tag:theconversation.com,2011:/au/topics/friction-7749/articlesFriction – The Conversation2023-12-14T13:10:42Ztag:theconversation.com,2011:article/2154062023-12-14T13:10:42Z2023-12-14T13:10:42ZLighting a fire using friction requires an understanding of some physics principles − but there are ways to make the process easier<figure><img src="https://images.theconversation.com/files/565288/original/file-20231212-25-x4dxir.jpg?ixlib=rb-1.1.0&rect=24%2C0%2C4001%2C3017&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Humans have been making fire by friction for centuries, but it's not easy.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/campfire-on-the-shores-of-the-chesapeake-bay-royalty-free-image/1402162981?phrase=campfire&adppopup=true">Cyndi Monaghan/Moment via Getty Images</a></span></figcaption></figure><p>Humans have been making <a href="https://www.primitiveways.com/fire_Baugh.html">fire using friction</a> for <a href="https://www.britannica.com/science/fire-combustion">thousands of years</a>, with evidence of its use found in archaeological records <a href="https://www.gutenberg.org/ebooks/53531">across different cultures</a> worldwide. </p>
<p>Fire by friction is a testament to human ingenuity, contributing to the development of early technology and a later understanding of physics, chemistry and heat transfer.</p>
<p>Making fire, one of the <a href="https://theconversation.com/early-humans-used-fire-to-permanently-change-the-landscape-tens-of-thousands-of-years-ago-in-stone-age-africa-158574">key discoveries in human history</a>, has played a <a href="https://doi.org/10.1002/evan.20275">pivotal role in human evolution</a>, providing warmth, light, protection from predators, a <a href="https://youtu.be/qv6kcj6Uv2Y?si=xEV7rK-k2U9GXRtK">means to cook</a> and the ability to migrate into more hostile climates. </p>
<p>I’m an <a href="https://udayton.edu/directory/graduate/duncan_bradley.php">engineering professor</a>, avid outdoorsman and <a href="https://firecrafter38.wildapricot.org/">Minisino Firecrafter</a> who’s been studying and practicing fire by friction for many years. It’s a great way to explore <a href="https://ecommons.udayton.edu/cgi/viewcontent.cgi?article=1418&context=ece_fac_pub">key science concepts</a> while engaging in a practice that humans have been performing for millennia.</p>
<h2>Ember, flame, fire</h2>
<p>Fire by friction relies on <a href="https://energyeducation.ca/encyclopedia/Friction">the conversion of</a> mechanical energy into thermal energy through friction. Friction is the force <a href="https://www.britannica.com/science/friction">of resistance</a> between two surfaces when they slide, or attempt to slide, past one another. </p>
<p>There are many ways to create fire by friction, but the most common and easiest to learn uses <a href="https://www.jonsbushcraft.com/bowdrill%20tutorial.htm">a bow drill set</a>.</p>
<p>A bow drill set consists of a thin spindle, a hearth board, a lightly curved bow, to which a bow cord is attached, and a “thunderhead” or bearing block, which is a stone or block of hard wood with a natural or carved divot used to press down on the top of the spindle. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/565523/original/file-20231213-31-9q62ua.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A set of wood tools, including a long stick with a cord attached, a small stick, a piece of wood with grooves carved into it, a pile of dried grass, and a small, triangle-shaped stone." src="https://images.theconversation.com/files/565523/original/file-20231213-31-9q62ua.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/565523/original/file-20231213-31-9q62ua.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/565523/original/file-20231213-31-9q62ua.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/565523/original/file-20231213-31-9q62ua.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/565523/original/file-20231213-31-9q62ua.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/565523/original/file-20231213-31-9q62ua.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/565523/original/file-20231213-31-9q62ua.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A bow drill set, made entirely from materials found outdoors. From bottom left to top right is a tinder bundle, made from the inner bark of a cottonwood tree and some red cedar bark, a stone thunderhead, a honeysuckle bow with a cord made from dogbane fibers, a goldenrod spindle and a white pine hearth board.</span>
<span class="attribution"><span class="source">Bradley Duncan</span></span>
</figcaption>
</figure>
<p>First, the firemaker wraps the bow cord tightly around the spindle and uses it to <a href="https://www.youtube.com/watch?v=S5h5tSAYPcw">rapidly spin</a> the spindle against the hearth board, while simultaneously pressing down with the thunderhead. </p>
<p>Similar to how your hands become warmer when you vigorously rub them together, friction causes a rapid increase in temperature where the spindle meets the hearth board. This drives away any residual moisture. The wood also heats up mostly in the <a href="https://en.wikipedia.org/wiki/Charcoal">absence of oxygen</a>, resulting in <a href="https://en.wikipedia.org/wiki/Charring">charring</a>, a chemical process from incomplete combustion. What’s left over is <a href="https://www.britannica.com/science/charcoal">mostly carbon</a>. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/S5h5tSAYPcw?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The friction of the spindle against the hearth board creates heat – kind of like how your hands warm up when you rub them together.</span></figcaption>
</figure>
<p>As the spindle continues to spin, it grinds away the charred wood to form a small pile of <a href="https://www.youtube.com/watch?v=GzLvqCTvOQY">charcoal</a> dust. As the dust pile grows, it will eventually coalesce and ignite to form an ember. </p>
<p>The ember’s ignition point depends on a variety of factors, including the type of wood, the temperature and the humidity. <a href="https://www.primitiveways.com/fire_Baugh2.html">Experiments often yield</a> ignition temperatures in the range of 650-800 degrees Farenheit (340-430 degrees Celsius), with the most reliable estimates on the order of 700 degrees F (370 degrees C). Getting to this temperature is essential to create an ember and start the fire.</p>
<p>After an ember forms, the firemaker then transfers it to a tinder bundle made of dry leaves or grass, dead tree bark or other fibrous organic materials. The firemaker blows into the tinder bundle to further <a href="https://gearuphiking.com/why-does-blowing-on-a-fire-make-it-burn-better/">raise the temperature</a> by increasing oxygen flow. </p>
<p>Eventually, the <a href="https://www.youtube.com/watch?v=ibaMy_WvhE0">tinder bursts into flame</a>, after which the firemaker can kindle it into a larger fire. Young fires are usually fragile – if the firemaker doesn’t provide them with enough fuel, air flow and protection from the wind and rain, they can go out.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/B0E4PX3e3RE?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The smoke you see rising from a fire results from incomplete combustion.</span></figcaption>
</figure>
<h2>Work smarter, not harder</h2>
<p>Understanding <a href="https://ecommons.udayton.edu/cgi/viewcontent.cgi?article=1418&context=ece_fac_pub">the physics of fire by friction</a> and the <a href="https://www.youtube.com/watch?v=8zEC1qSjKjg">different variables involved</a> can make a big difference and help the fire start more quickly with less effort.</p>
<p>First, <a href="https://www.youtube.com/watch?v=gnsDMEOVShQ">keep it small</a>. Firemakers should make bow drill sets carved from standing dead, dry tree limbs maybe an inch or so (2.5 centimeters) in diameter. Optimal spindles have diameters between three-eighths of an inch and a half-inch (1-1.25 cm). </p>
<p>How fast the friction force generates heat is directly proportional to how fast the firemaker bows, on average, and is independent of the diameter of the spindle. So, the faster you move the bow, the more heat you will create, <a href="https://ecommons.udayton.edu/cgi/viewcontent.cgi?article=1418&context=ece_fac_pub">regardless of the spindle’s size</a>. </p>
<p>But because they have smaller cross-sectional areas, thin spindles increase the heat density at the spindle-hearth board interface, which is where the ember forms and ignites. By concentrating the heat in a smaller area at this interface, thin spindles reduce the time and effort required to form and ignite an ember.</p>
<p>Dry, unpigmented, medium-density woods – elm, poplar and cottonwood are some examples – will work well for the spindle and the hearth board. I’ve tested lots of wood types and found that, with a few exceptions, wood hardness <a href="https://www.youtube.com/watch?v=2VleagqhZjU">mostly doesn’t matter</a>. </p>
<p>I’ve also found that mature wildflower stalks – harvested fresh and allowed to dry out – work well as spindles. Tall, woody wildflowers like <a href="https://www.youtube.com/watch?v=Rz_wiVyBG8c">goldenrod</a>, <a href="https://www.youtube.com/watch?v=W7hpmbdu2n0">ironweed</a>, <a href="https://www.youtube.com/watch?v=9sLKdif9BGQ">teasel</a>, <a href="https://www.youtube.com/watch?v=q33d__UYZ8c">mullein</a> and the like can produce embers in seconds. If time permits, you can even make a bow cord with <a href="https://www.youtube.com/watch?v=1R1wC_mLPVo">natural fibers</a> extracted from flax, dogbane or nettle plants commonly found in the woods.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/565286/original/file-20231212-17-7tuxrb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A yellow goldenrod flower, with green leaves." src="https://images.theconversation.com/files/565286/original/file-20231212-17-7tuxrb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/565286/original/file-20231212-17-7tuxrb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/565286/original/file-20231212-17-7tuxrb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/565286/original/file-20231212-17-7tuxrb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/565286/original/file-20231212-17-7tuxrb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/565286/original/file-20231212-17-7tuxrb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/565286/original/file-20231212-17-7tuxrb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The thick, woody stalks from wildflowers like goldenrod can work as effective spindles.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/solidago-goldenrod-royalty-free-image/157186277?phrase=goldenrod&adppopup=true">Solidago/E+ via Getty Images</a></span>
</figcaption>
</figure>
<h2>The fire-making process</h2>
<p>The <a href="https://www.youtube.com/watch?v=8zEC1qSjKjg">key variables</a> the firemaker can control during the bowing process are the speed at which they’re moving the bow and how much pressure they’re applying to the spindle via the thunderhead. Start by seating the spindle tip into a <a href="http://paulkirtley.co.uk/wp-content/uploads/2011/01/notch.jpg">notched divot</a> carved into the hearth board. Then move the bow slowly until you get your balance. </p>
<p>Initially press down with the thunderhead just hard enough <a href="https://www.ars.usda.gov/northeast-area/wyndmoor-pa/eastern-regional-research-center/docs/biomass-pyrolysis-research-1/what-is-pyrolysis/">for pyrolysis to begin</a>. Pyrolysis happens when heat causes organic material to decompose without oxygen. You’ll know when pyrolysis starts because you’ll see smoke rising from the spindle-hearth board interface. </p>
<p>Then, begin to increase your bow speed until you are bowing as rapidly as you can sustain for a minute or so. Don’t hold your breath, and use bow strokes as long as you can manage without compromising bow speed. The time it takes to form an ember decreases the faster you bow, though the length of your <a href="https://www.youtube.com/watch?v=8zEC1qSjKjg">stroke doesn’t matter</a>.</p>
<p>As speed increases, begin to increase the pressure you’re putting on the spindle, stopping when the increased friction begins to affect your ability to sustain a rapid bow speed. With good materials you’ll likely have a nice ember in well under a minute.</p>
<p>While modern technology has largely replaced primitive methods, fire by friction continues to be a source of fascination and a testament to human ingenuity. Understanding this process not only enriches humanity’s connection to the ancient past, but it also underscores how physics comes into play throughout daily life.</p><img src="https://counter.theconversation.com/content/215406/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Bradley Duncan does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>You may have seen contestants on reality shows like “Survivor” make fire using friction, but do you know the physics behind the process?Bradley Duncan, Professor of Electrical and Computer Engineering, University of DaytonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1764632022-02-14T18:45:15Z2022-02-14T18:45:15ZThe slippery science of Olympic curling: we still don’t know how it works<p>Australia’s first ever Olympic curling team <a href="https://www.smh.com.au/sport/curlers-forced-into-heartbreaking-olympics-withdrawal-20220206-p59u6k.html">scored an historic win but missed the medal podium</a> at the 2022 Winter Olympic Games in Beijing. It was a remarkable performance for a team <a href="https://www.smh.com.au/sport/one-man-s-ridiculous-push-to-turn-australia-into-a-curling-nation-20220205-p59u2x.html">lacking any dedicated curling facilities</a> at home. </p>
<p>And that’s important, because it is the special properties of curling ice that allow the heavy curling stones to glide and curve in ways that seem to defy physics. In fact, scientists are still not sure what puts the “curl” in curling. </p>
<h2>Chess on ice</h2>
<p>Curling’s origins date back to 16th-century Scotland, making it <a href="https://worldcurling.org/about/history/">one of the world’s oldest team sports</a>. Like golf – invented around the same time in the same part of the world – curling seems both amusingly pointless and deceptively simple to the untrained eye. </p>
<p>It has been called “chess on ice”, although to many Australians it most resembles frozen lawn bowls. Athletes take turns sliding circular 20-kilogram granite stones along the ice toward the centre of a horizontal target 28 metres away. Teams are awarded points for getting their stones closest to the centre of the target, or “house”. </p>
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<p>
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Read more:
<a href="https://theconversation.com/why-curling-is-so-gripping-to-watch-91857">Why curling is so gripping to watch</a>
</strong>
</em>
</p>
<hr>
<h2>Slippery science</h2>
<p>The slippery science behind curling starts with the ice itself. Curling ice must be perfectly flat – far flatter than a typical ice hockey rink – and is sprayed with droplets of water before each game to produce a pebbled surface. This minimises the contact area between the ice and the heavy curling stone.</p>
<p>Curling stones also have a concave lower surface – like the bottom of a beer bottle – that further reduces the contact area between the stone and the ice. The effect is to increase the pressure at the base of the stone, partially melting the ice and reducing friction in a similar way to how ice skates work. </p>
<p>Uniquely among Olympic sports, curling players can change the path of the stone after it has been “thrown”. This is achieved by vigorously sweeping the ice in front of the stone with special brooms that warm the ice and reduce friction, allowing the stone to travel farther and straighter along its path. </p>
<p>Deciding when, where, and how hard to sweep has a big influence on the stone’s trajectory; so naturally it is accompanied by a great deal of enthusiastic yelling. </p>
<h2>Give it a spin</h2>
<p>By adding a small amount of spin, skilled players can make their stone “curl” along a curving path to block an opponent’s stone or knock it out of the way. Even a small amount of rotation can deflect the path of the curling stone by as much as a metre and a half. How exactly the curling stone does this is <a href="https://youtu.be/7CUojMQgDpM">something of a puzzle</a>.</p>
<p>Let’s start with a (literal) tabletop experiment. Slide an upturned glass along a table, adding a little spin as it leaves your hand. With a little practice (and perhaps a few replacement glasses) you will be able to make the glass trace a curving path across the table, deflecting to the left when you spin it clockwise or to the right when you spin it anticlockwise. </p>
<p>The reason for this is explained by a branch of science called <a href="https://en.wikipedia.org/wiki/Tribology">tribology</a>, which studies the effect of friction on moving and sliding objects. </p>
<p>As the glass spins, it rubs against the table top, generating friction that tries to slow down the rotation of the glass. The friction forces are directed <em>opposite</em> to the direction of motion: for a clockwise-rotating glass, friction will be directed to the left at the front of the glass and to the right at the back of the glass. </p>
<p>When the spinning glass slides across the table, it leans forward slightly in the direction of travel, pushing the front lip of the glass down a little harder on the table than the trailing lip. The extra pressure generates extra friction at the front compared to the back. The resulting imbalance of friction forces causes the glass to deflect in the direction of stronger friction – to the left in the case of a clockwise-rotating glass. </p>
<h2>A twist in the tale</h2>
<p>But curling stones behave in exactly the opposite way: a clockwise rotation causes the stone to deflect to the right, not the left. For a long time, scientists assumed this was because of an effect called <a href="https://cdnsciencepub.com/doi/abs/10.1139/p96-095?casa_token=I-BZPmap9awAAAAA:-s9g73B59PpwUroGYk8Fbv46XwmGASvq9KNqyEtJItlIx67tUAjLNWkezzM74Wfn3EjxUJwexw8">asymmetrical friction</a>. </p>
<p>The theory goes like this: like a glass pushed across a table, a curling stone leans forward slightly. The extra pressure at the front of the stone partially melts the ice at the leading edge, creating a thin film of water that <em>reduces</em> the friction at the front of stone compared with the back. </p>
<p>The curling stone will still deflect in the direction of stronger friction. But in this case, it is the <em>trailing edge</em> that wins, resulting in a deflection to the <em>right</em> rather than the left, for a clockwise-rotating stone. </p>
<h2>Scratch that</h2>
<p>Like many theories, this explanation was widely accepted until someone got around to actually testing it. In 2012, a team at Uppsala University in Sweden <a href="https://link.springer.com/article/10.1007/s11249-013-0135-9">made detailed calculations</a> of the friction forces acting on a sliding stone. </p>
<p>The problem they found is that curling stones rotate quite slowly, only completing a couple of turns before coming to a stop. This spin is far too small to cause a sideways deflection of a metre or more. Even odder, more rotation does not lead to more curl – in fact, spin a stone too hard and it won’t curl at all. Asymmetrical friction cannot explain such behaviour. </p>
<p>The researchers <a href="https://www.sciencedirect.com/science/article/abs/pii/S0043164813000732">used an electron microscope</a> to look more closely at the ice under a curling stone. They discovered that the front edge of the stone leaves behind miniscule scratches on the ice in the direction of rotation. These scratches act as a guide for the back edge of the stone, causing the stone to deflect in the direction of rotation. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/446136/original/file-20220213-21-n6eucz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/446136/original/file-20220213-21-n6eucz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/446136/original/file-20220213-21-n6eucz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=479&fit=crop&dpr=1 600w, https://images.theconversation.com/files/446136/original/file-20220213-21-n6eucz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=479&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/446136/original/file-20220213-21-n6eucz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=479&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/446136/original/file-20220213-21-n6eucz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=603&fit=crop&dpr=1 754w, https://images.theconversation.com/files/446136/original/file-20220213-21-n6eucz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=603&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/446136/original/file-20220213-21-n6eucz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=603&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Curling stones make microscopic scratches in the pebbled surface of the ice - and according to one theory, these scratches deflect the stone’s path to the left or right.</span>
<span class="attribution"><a class="source" href="https://www.sciencedirect.com/science/article/pii/S0043164813000732?via%3Dihub"> H. Nyberg, et al., Wear (2013)</a></span>
</figcaption>
</figure>
<p>The Swedish team then showed that, using this “scratch-guide” mechanism, they could “steer” the sliding stones by adding artificial scratches to the ice in different directions. In one experiment, a stone was made to travel along a zigzag path by laying down scratches in alternating directions. </p>
<p>Their findings ignited a <a href="https://www.newyorker.com/tech/annals-of-technology/physicists-still-dont-know-what-puts-the-curl-in-curling">minor controversy</a> in the admittedly niche world of curling physics. </p>
<p>Competing theories have been proposed, including the <a href="https://doi.org/10.1139/cjp-2016-0466">pivot-slide model</a>, the <a href="https://doi.org/10.5331/bgr.28.1">evaporation-abrasion model</a>, and the <a href="https://cdnsciencepub.com/doi/abs/10.1139/p02-072?casa_token=lhb8AAtp71QAAAAA:gUi_J0hlhR3AVyukR2--uTYTH77ZGQ0n4mbD1i5IrGxPsEyLuxNIge4R6j_YZnqW1pkw8G9fEVw">snowplow model</a>. </p>
<p>In 2020, a Japanese team attempted to clear things up by <a href="https://www.nature.com/articles/s41598-020-76660-8">systematically testing each theory</a> in a curling hall using sophisticated motion-tracking equipment, a laser scanning microscope, and some sheets of sandpaper to modify the surface of the curling stone.</p>
<p>However, no clear winner emerged. When it comes to the science of curling, it appears we are just scratching the surface.</p><img src="https://counter.theconversation.com/content/176463/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Shane Keating does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The game of curling is centuries old, but exactly what makes the stones curl as they slide over the ice is still a mystery.Shane Keating, Senior Lecturer in Mathematics and Oceanography, UNSW SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1759852022-02-04T19:59:53Z2022-02-04T19:59:53ZThe high-speed physics of how bobsled, luge and skeleton send humans hurtling faster than a car on the highway<figure><img src="https://images.theconversation.com/files/444394/original/file-20220203-19-xcns7h.jpg?ixlib=rb-1.1.0&rect=419%2C1373%2C2781%2C1718&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Bobsled, luge and skeleton athletes descend twisting, steep tracks at speeds upward of 80 mph (130 kmh).</span> <span class="attribution"><a class="source" href="https://newsroom.ap.org/detail/VancouverOlympicsLuge/04f06c3e67b7419fb8c7501593eb84f9/photo?Query=luge%20olympic&mediaType=photo&sortBy=&dateRange=Anytime&totalCount=4131&currentItemNo=49">AP Photo/Sergey Ponomarev</a></span></figcaption></figure><p>Speed alone may be the factor that draws many sports fans to the <a href="https://www.beijing2022.cn/en/olympics_/bobsleigh.htm">bobsled, luge and skeleton events</a> at this year’s Beijing Winter Olympics. But beneath the thrilling descents of the winding, ice-covered track, a myriad of concepts from physics are at play. It is how the athletes react to the physics that ultimately determines the fastest runs from the rest of the pack.</p>
<p><a href="https://scholar.google.com/citations?hl=en&user=eHzYy_EAAAAJ">I study the physics of sports</a>. Much of the excitement of a luge run is easy to miss – the athletes’ movements are often too small to notice as they fly by looking like nothing more than a blur on your television. It would be easy to assume that the competitors are simply falling or sliding down a track at the whim of gravity. But that thought merely scratches the surface of all the subtle physics that go into a gold-medal-winning performance.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/444397/original/file-20220203-21-1bpy1ig.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="An aerial view of a large twisting covered track." src="https://images.theconversation.com/files/444397/original/file-20220203-21-1bpy1ig.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/444397/original/file-20220203-21-1bpy1ig.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=306&fit=crop&dpr=1 600w, https://images.theconversation.com/files/444397/original/file-20220203-21-1bpy1ig.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=306&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/444397/original/file-20220203-21-1bpy1ig.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=306&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/444397/original/file-20220203-21-1bpy1ig.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=385&fit=crop&dpr=1 754w, https://images.theconversation.com/files/444397/original/file-20220203-21-1bpy1ig.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=385&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/444397/original/file-20220203-21-1bpy1ig.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=385&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Tracks for sliding events – like the Olympic track from the 2018 Pyeongchang Winter Olympics – drop hundreds of feet and feature many tight turns.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Olympic_Sliding_Centre#/media/File:Alpensia_20170202_05_(32619189236).jpg">Korean Culture and Information Service via Wikimedia Commons</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-sa/4.0/">CC BY-NC-SA</a></span>
</figcaption>
</figure>
<h2>Gravity and energy</h2>
<p>Gravity is what powers the sleds down the ice-covered tracks in bobsled, luge and skeleton events. The big-picture physics is simple – start at some height and then fall to a lower height, letting gravity accelerate athletes to speeds <a href="https://www.si.com/olympics/2018/02/13/how-fast-does-luge-go-speed-velocity">approaching 90 mph</a> (145 kph). </p>
<p>This year’s races are taking place at the <a href="https://www.beijing2022.cn/en/olympics_/bobsleigh.htm">Yanqing National Sliding Center</a>. The track is roughly a mile long (1.6 km), drops 397 feet of elevation (121 meters) – with the steepest section being an incredible 18% grade – and <a href="https://www.ibsf.org/en/tracks/track/700012/Yanqing">comprises 16 curves</a>.</p>
<p>Riders in the sledding events reach their fast speeds because of the conversion of gravitational potential energy into kinetic energy. Gravitational potential energy represents stored energy and increases as an object is raised farther from Earth’s surface. The potential energy is converted to another form of energy once the object starts falling. Kinetic energy is the energy of motion. The reason a flying baseball will shatter the glass if it hits a window is that the ball transfers its kinetic energy to the glass. Both gravitational potential energy and kinetic energy increase as weight increases, meaning there is more energy in a four-person bobsled team than there is in a one-person luge or skeleton for a given speed.</p>
<p>Racers are dealing with a lot of kinetic energy and strong forces. When athletes enter a turn at 80 mph (129 kph) they experience accelerations that can reach <a href="https://www.technogym.com/us/newsroom/luge-courage-and-high-speeds/">five times that of normal gravitational acceleration</a>. Though bobsled, luge and skeleton may look easy, in reality they are anything but.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/444403/original/file-20220203-15-16gh0zf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A luge racer lying on his back in an aerodynamic pose." src="https://images.theconversation.com/files/444403/original/file-20220203-15-16gh0zf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/444403/original/file-20220203-15-16gh0zf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=460&fit=crop&dpr=1 600w, https://images.theconversation.com/files/444403/original/file-20220203-15-16gh0zf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=460&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/444403/original/file-20220203-15-16gh0zf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=460&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/444403/original/file-20220203-15-16gh0zf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=578&fit=crop&dpr=1 754w, https://images.theconversation.com/files/444403/original/file-20220203-15-16gh0zf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=578&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/444403/original/file-20220203-15-16gh0zf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=578&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Racers need to be as aerodynamic as possible to minimize drag and go faster.</span>
<span class="attribution"><a class="source" href="https://newsroom.ap.org/detail/VancouverOlympicsLuge/7ae2fa592a92471387554d0f45f29ccf/photo?Query=luge%20olympic&mediaType=photo&sortBy=&dateRange=Anytime&totalCount=4131&currentItemNo=97">AP Photo/Ricardo Mazalan</a></span>
</figcaption>
</figure>
<h2>Aerodynamics</h2>
<p>Most tracks are around a mile long (1.6 km), and the athletes cover that distance in just under a minute. Final times are calculated by adding four runs together. The difference between the gold medal and silver medal in the men’s singles luge at the 2018 Winter Olympics <a href="http://www.fil-luge.org/cdn/uploads/lugmsingles-c73b2-1-0.pdf">was just 0.026 seconds</a>. Even tiny mistakes made by the best athletes in the world can cost a medal.</p>
<p>All the athletes start at the same height and go down the same track. So the difference between gold and a disappointing result comes not from gravity and potential energy, but from a fast start, being as aerodynamic as possible and taking the shortest path down the track. </p>
<p>While gravity pulls the athletes and their sleds downhill, they are constantly colliding with air particles that create a force called air drag, which pushes back on the athletes and sleds in a direction opposite to their velocity. The more aerodynamic an athlete or team is, the greater the speed.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/444401/original/file-20220203-15-1b14kwq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A team of bobsled racers going around a corner." src="https://images.theconversation.com/files/444401/original/file-20220203-15-1b14kwq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/444401/original/file-20220203-15-1b14kwq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=377&fit=crop&dpr=1 600w, https://images.theconversation.com/files/444401/original/file-20220203-15-1b14kwq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=377&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/444401/original/file-20220203-15-1b14kwq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=377&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/444401/original/file-20220203-15-1b14kwq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=473&fit=crop&dpr=1 754w, https://images.theconversation.com/files/444401/original/file-20220203-15-1b14kwq.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=473&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/444401/original/file-20220203-15-1b14kwq.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=473&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Bobsled teams must tuck themselves behind the leading edge of the sled to avoid the oncoming air.</span>
<span class="attribution"><a class="source" href="https://newsroom.ap.org/detail/APTOPIXVancouverOlympicsBobsled/e41cf5b02a8b4a98ba1ad279d1c31190/photo?Query=bobsleigh%20olympic&mediaType=photo&sortBy=&dateRange=Anytime&totalCount=5034&currentItemNo=4">AP Photo/Andrew Medichini</a></span>
</figcaption>
</figure>
<p>To minimize drag from the air, luge riders – who are face up – lie as flat as possible. Downward-facing skeleton riders do the same. Whether in a team of two or four, bobsled riders stay tucked tightly inside the sled to reduce the area available for air to smash into. Any body positioning mistakes can make athletes less aerodynamic and lead to tiny increases in time that can cost them a medal. And these mistakes are tough to correct at the high accelerations and forces of a run.</p>
<h2>The shortest way down</h2>
<p>Besides being as aerodynamic as possible, the other major difference between a fast and a slow run is the path riders take. If they minimize the total length taken by their sleds and avoid zigzagging across the track, riders will cover less distance. In addition to simply not having to go as far to cross the finish line, shortening the path means facing less drag from air and losing less speed from friction with the track.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/444400/original/file-20220203-27-9hvavp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A skeleton racer running with his sled at the start of a race." src="https://images.theconversation.com/files/444400/original/file-20220203-27-9hvavp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/444400/original/file-20220203-27-9hvavp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/444400/original/file-20220203-27-9hvavp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/444400/original/file-20220203-27-9hvavp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/444400/original/file-20220203-27-9hvavp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/444400/original/file-20220203-27-9hvavp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/444400/original/file-20220203-27-9hvavp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Skeleton racers don’t have a means of directly controlling the runners, so they must use subtle body movements to flex the sled and initiate turns.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Nozomi_Komuro_start_LP_World_Cup_2017_(1_of_1).jpg#/media/File:Nozomi_Komuro_start_LP_World_Cup_2017_(1_of_1).jpg">121a0012 via Wikimedia Commons</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Fans often miss the subtleties involved in turning and steering. The sleds for all the events sit on <a href="http://www.teamusa.org/%7E/media/USA_Luge/Documents/IRO_2014_AT_English.pdf">steel blades called runners</a>. Bobsleds have two sets of runners that make contact with the ice. The front rider pulls on <a href="https://www.rulesofsport.com/sports/bobsleighing.html">rings attached to pulleys that turn the front runners</a>. Runners on luge sleds have curved bows at the front where riders place their calves. By moving their head and shoulders or flexing their calves, athletes can turn the luge. Skeleton riders lack these controls and must <a href="http://www.ibsf.org/images/documents/downloads/2015_International_Rules_SKELETON.pdf">flex the sled</a> itself using their shoulders and knee to initiate a turn. Even a tiny head movement can cause the skeleton to move off the optimal path.</p>
<p>All of these subtle movements are hard to see on television, but the consequences can be large – oversteering may lead to collisions with the track wall or even crashes. Improper steering may lead to bad turns that cost riders time.</p>
<p>Though it may appear that the riders simply slide down the icy track at great speeds after they get going, there is a lot more going on. Viewers will have to pay close attention to the athletes on those fast-moving sleds to detect the interesting facets of physics in action.</p>
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<p class="fine-print"><em><span>John Eric Goff does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>It may look like athletes in bobsled, luge and skeleton simply grab a sled and hang on until the bottom, but high-speed physics and tiny motions mean the difference between gold and a crash.John Eric Goff, Professor of Physics, University of LynchburgLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1624152021-06-15T12:25:22Z2021-06-15T12:25:22ZSticky baseballs: Explaining the physics of the latest scandal in Major League Baseball<figure><img src="https://images.theconversation.com/files/406219/original/file-20210614-126247-1yxj5ll.jpg?ixlib=rb-1.1.0&rect=30%2C39%2C2907%2C1916&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">It used to be spit balls, but now sticky baseballs are giving pitchers an advantage.</span> <span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Baseball.jpg#/media/File:Baseball.jpg">Tage Olsin</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Cheating in baseball is as old as the game itself, and pitchers’ modifying the ball’s surface is part of that <a href="https://www.simonandschuster.com/books/The-Neyer-James-Guide-to-Pitchers/Bill-James/9780743261586">long history</a>. Adding to the lore of cheating is a <a href="https://nypost.com/2021/06/09/questions-and-answers-surrounding-mlbs-sticky-stuff-problem/">new scandal</a> involving pitchers who may be applying sticky substances – what players refer to as “sticky stuff” – to baseballs.</p>
<p>Major League hitters are striking out this season <a href="https://blogs.fangraphs.com/lets-take-another-stab-at-unpacking-the-rising-strikeout-rate/">nearly one in every four times they step to the plate</a>, compared with one in six times in 2005. </p>
<p><a href="https://scholar.google.com/citations?user=eHzYy_EAAAAJ&hl=en&oi=ao">As a sports physicist</a> and longtime baseball fan, I’ve been intrigued by <a href="https://www.thescore.com/mlb/news/2180019">news reports</a> that applying sticky substances to balls can make pitches spin faster. And if pitchers can throw their fastballs, curveballs and sliders with more spin than in previous years, their pitches will be tougher to hit. How does science explain all this?</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/406220/original/file-20210614-125373-10ppylh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A woman struggling to open a jar full of preserves." src="https://images.theconversation.com/files/406220/original/file-20210614-125373-10ppylh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/406220/original/file-20210614-125373-10ppylh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/406220/original/file-20210614-125373-10ppylh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/406220/original/file-20210614-125373-10ppylh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/406220/original/file-20210614-125373-10ppylh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/406220/original/file-20210614-125373-10ppylh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/406220/original/file-20210614-125373-10ppylh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">When you can’t get a jar open, increasing friction between your hand and the lid can help.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/the-lid-is-really-tight-royalty-free-image/175213030?adppopup=true">Steex/iStock via Getty Images Plus</a></span>
</figcaption>
</figure>
<h2>Sticky stuff increases friction and torque</h2>
<p>If you want to understand what all the sticky fuss is about, you need to know some friction basics.</p>
<p>You’ve surely tried to unscrew a lid from a stubborn jar. If there isn’t enough friction between your fingers and the lid, you may not be able to exert enough torque – the rotational analog of force – to get the lid to turn. One way to get more torque on the lid is to increase the frictional force. In my home, we keep a circular piece of rubber to increase friction and help open tough jars. </p>
<p>Pitchers want more friction between their fingers and the baseball, and they are supposedly using some interesting substances to accomplish this. According to a <a href="https://www.si.com/mlb/2021/06/04/sticky-stuff-is-the-new-steroids-daily-cover">recent Sports Illustrated article</a>, “pitchers have begun experimenting with drumstick resin and surfboard wax.” “They use Tyrus Sticky Grip, Firm Grip spray, Pelican Grip Dip stick and Spider Tack, a glue intended for use in World’s Strongest Man competitions and whose advertisements show someone using it to lift a cinder block with his palm.” That article noted one instance of a ball so sticky players could see fingerprints on it, and another story in which a ball could be stuck to a person’s open hand with his palm facing the ground. All of these sticky substances would increase friction and thus give pitchers a better grip on the ball.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/406222/original/file-20210614-131717-1e75bef.gif?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A spinning cylinder with smoke helping to visualize the uneven air currents." src="https://images.theconversation.com/files/406222/original/file-20210614-131717-1e75bef.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/406222/original/file-20210614-131717-1e75bef.gif?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=323&fit=crop&dpr=1 600w, https://images.theconversation.com/files/406222/original/file-20210614-131717-1e75bef.gif?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=323&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/406222/original/file-20210614-131717-1e75bef.gif?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=323&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/406222/original/file-20210614-131717-1e75bef.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=406&fit=crop&dpr=1 754w, https://images.theconversation.com/files/406222/original/file-20210614-131717-1e75bef.gif?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=406&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/406222/original/file-20210614-131717-1e75bef.gif?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=406&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Topspin creates a wake of air that pushes a ball down, as seen in the image above where air is flowing right to left past the metal cylinder in the center that is spinning clockwise.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Magnus-anim-canette.gif#/media/File:Magnus-anim-canette.gif">MatSouffNC858s/WkimediaCommons</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>More spin makes pitches harder to hit</h2>
<p>Today’s sticky fingers are the latest attempts by players to gain an <a href="https://registration.mlbpa.org/pdf/MajorLeagueRules.pdf">unfair</a> <a href="https://www.mlb.com/news/the-unwritten-rules-of-baseball">advantage</a>. But how does sticky stuff make a pitch harder to hit? It helps increase spin rate. </p>
<p>Unless a pitcher throws a knuckleball, which has very little spin, all baseballs are spinning at well over 1,000 revolutions per minute when they leave pitchers’ hands. That spin creates a force – let’s call it the spin force – that causes baseballs to move and curve in ways that can throw off hitters.</p>
<p>As air smashes into a moving baseball, it doesn’t wrap completely around the ball – it separates off the surface before reaching the back of the ball. Think of water flowing along the sides of a moving boat. The water doesn’t smoothly wrap around the back of the boat – there is a wake of turbulent water behind it. But when a rudder turns the boat, the wake moves off to one side. Newton’s third law says that if the boat pushes water in one direction, water has to push the boat in the opposite direction, causing the boat to turn.</p>
<p>[<em><a href="https://theconversation.com/us/newsletters/science-editors-picks-71/?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=science-corona-important">The Conversation’s most important coronavirus headlines, weekly in a science newsletter</a></em>]</p>
<p>The same idea applies to a spinning baseball. If the baseball is spinning, the wake of air behind the ball will be asymmetric. So the spin force pushes the ball in the opposite direction from which the wake of air is pushed.</p>
<p>Consider an overhand curveball. In this pitch, a Major League Baseball pitcher pulls down on the front of the ball when he releases it, generating topspin. A top-spinning curveball pushes air upward off the back of the ball, just like a wake coming off one side of a boat. Because the ball pushes the wake of air upward, the air’s force on a curveball is downward. Curveballs thus experience a push downward on their way to the plate, all thanks to the spin force.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/406224/original/file-20210614-126665-1iiqqfu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A baseball player swinging and missing a pitch." src="https://images.theconversation.com/files/406224/original/file-20210614-126665-1iiqqfu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/406224/original/file-20210614-126665-1iiqqfu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/406224/original/file-20210614-126665-1iiqqfu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/406224/original/file-20210614-126665-1iiqqfu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/406224/original/file-20210614-126665-1iiqqfu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/406224/original/file-20210614-126665-1iiqqfu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/406224/original/file-20210614-126665-1iiqqfu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The extra spin from sticky stuff could make a baseball move 2 inches more compared with pitches in previous years.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/news-photo/taylor-walls-of-the-tampa-bay-rays-swings-at-a-pitch-during-news-photo/1323047032?adppopup=true">Douglas P. DeFelice/Contributor via Getty Images Sport</a></span>
</figcaption>
</figure>
<h2>How effective is sticky stuff?</h2>
<p>Here is where the alleged cheating comes in. </p>
<p>As with pitchers in the past, a Major League pitcher today could put sticky stuff on his fingers in the locker room, stick some to his uniform or even get some from a teammate. The substances starring in the current scandal would help create more spin. A good pitcher can throw a curveball at 85 mph with a spin rate of 2,400 rpm with about 20 pounds of friction force between the pitcher’s fingers and the ball. <a href="https://baseballsavant.mlb.com/">Freely available pitch data</a> shows that some pitchers have increased their spin rate by about 400 rpm on curveballs compared with previous seasons. That’s a 17% increase in spin rate and requires a 17% increase – or an additional 3 pounds – of friction force coming from sticky substances.</p>
<p>For an overhand curveball, an extra 400 rpm of topspin can lead to more than 2 inches of additional vertical drop – which just happens to be the thickness of the sweet spot of a baseball bat. In other words, a Major League Baseball batter familiar with a pitcher’s curveball might swing where he thinks he’ll make great contact, except because the sticky stuff and extra spin the ball will cross the plate 2 inches lower than the batter expects. He’ll either miss the pitch or hit a weak grounder.</p>
<p>Strikeouts are happening at an <a href="https://calltothepen.com/2021/04/17/mlb-strikeouts-killing-game/">all-time high rate</a> and sticky stuff may be one of the culprits. Major League Baseball is already <a href="https://www.espn.com/mlb/insider/story/_/id/31596907/spider-tack-goo-cops-open-secret-answering-20-questions-mlb-foreign-substance-mess">contemplating</a> what to do about all the reports of sticky fingers. Umpires may soon periodically check pitchers during games. </p>
<p>But whatever the league decides, the cat-and-mouse game between players seeking enhanced performance and the league trying to catch them will continue, adding to the rich lore of cheating in baseball.</p><img src="https://counter.theconversation.com/content/162415/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>John Eric Goff does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Pitchers in Major League Baseball have been striking out more batters than ever, and some people say it’s because they’re adding sticky stuff to the balls.John Eric Goff, Professor of Physics, University of LynchburgLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1400662020-06-09T05:56:39Z2020-06-09T05:56:39ZCurious Kids: how would they bring the International Space Station back down to Earth?<figure><img src="https://images.theconversation.com/files/340465/original/file-20200609-176580-1qp5oqg.jpg?ixlib=rb-1.1.0&rect=34%2C34%2C4546%2C2603&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Shutterstock</span></span></figcaption></figure><hr>
<blockquote>
<p><strong>How would they bring the International Space Station back down to earth? Grace, age 7, Watson, ACT.</strong></p>
</blockquote>
<hr>
<p><a href="https://theconversation.com/au/topics/curious-kids-36782"><img src="https://images.theconversation.com/files/291898/original/file-20190911-190031-enlxbk.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=90&fit=crop&dpr=1" width="100%"></a></p>
<p>Hi Grace! The good news is we can bring the International Space Station back to Earth. The bad news is it will be a broken mess of melted metal. To understand why, we need to talk about a few other things first, including forces, orbits and gravity.</p>
<h2>How did the ISS get to space in the first place?</h2>
<p>The <a href="https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-the-iss-58.html">International Space Station</a> (or ISS for short) is about as big as a football field. It’s too big to send into space in one go. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/curious-kids-how-big-is-the-international-space-station-121442">Curious Kids: How big is the International Space Station?</a>
</strong>
</em>
</p>
<hr>
<p>It took 37 trips with NASA’s <a href="https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-the-space-shuttle-58.html">space shuttle</a> to carry most of the pieces 350km above Earth – that’s about the same height as 3,850 football fields standing up on end. It took astronauts more than ten years to put it all together up in space!</p>
<h2>Why doesn’t the ISS fall down to Earth?</h2>
<p>To understand how the ISS stays up there, we need to know about forces. </p>
<p>You can’t see force, but you can feel it. Forces can make things move, and stop them moving. </p>
<p>There are two different forces keeping the ISS in place. The first is gravity. Things that are very heavy such as planets, make a force that pulls smaller things (such as you) towards them. The reason we don’t float into the sky (and the reason you fall down again when you jump into the air) is because Earth’s gravity is always pulling us onto the ground. </p>
<p>When astronauts go far away from Earth, they float because they have escaped the force of gravity. But why doesn’t gravity pull the ISS back to Earth? </p>
<p>To answer that, we have to know about another force called centrifugal force. When things move in a circle, centrifugal force pushes them to the outside of the circle. You may have felt this while riding on a roundabout at the park, or when you’re in a car going around a corner and you get squashed against the door.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/340459/original/file-20200609-176554-197r984.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/340459/original/file-20200609-176554-197r984.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/340459/original/file-20200609-176554-197r984.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/340459/original/file-20200609-176554-197r984.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/340459/original/file-20200609-176554-197r984.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/340459/original/file-20200609-176554-197r984.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/340459/original/file-20200609-176554-197r984.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/340459/original/file-20200609-176554-197r984.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">When you’re pushed back on a roundabout (or carousel) swing at the park, you’re feeling centrifugal force.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/children-carousel-114125920">Shutterstock</a></span>
</figcaption>
</figure>
<p>The ISS moves in a circle around Earth at just the right speed. The centrifugal force pushing it away is exactly the same as the force of gravity pulling it in. This balance is called a stable orbit. And unless something happens to change it, it will continue.</p>
<h2>Can we bring the ISS back to Earth?</h2>
<p>In terms of when the ISS will actually be returned to Earth, we’re not sure yet. But it’s likely this will happen after five years from now.</p>
<p>When astronauts return to Earth from the ISS, they come back in the same small capsule that took them there. It can fit three people and not much else. It’s nowhere near big enough to fit in the ISS, even if we broke it into pieces. So what will happen when we no longer use the ISS? </p>
<p>Some parts may be kept in space to be used again in a new space station. But most of the ISS will return to Earth. To do this, the mission controllers (the people who run the ISS) will use rockets attached to the station to drive it closer to Earth. When it’s close enough, gravity will start to pull it in. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/curious-kids-where-does-the-oxygen-come-from-in-the-international-space-station-and-why-dont-they-run-out-of-air-82910">Curious Kids: Where does the oxygen come from in the International Space Station, and why don’t they run out of air?</a>
</strong>
</em>
</p>
<hr>
<p>Eventually it will hit the atmosphere (the layer of air around the Earth) and burst into flames as it burns up. This is due to another force called friction, which happens when two things try to slide past each other really fast. Friction makes things hot.</p>
<p>Once the ISS passes the atmosphere, it will likely crash into an empty part of the Pacific ocean called the “Oceanic Pole of Inaccessability”. This is one of the emptiest places on Earth, between New Zealand and Antarctica. Other space stations, such as the Russian <a href="https://history.nasa.gov/SP-4225/mir/mir.htm">Mir space station</a> are already there, four kilometres below the sea’s surface.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/340236/original/file-20200608-176550-1f5lfr1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/340236/original/file-20200608-176550-1f5lfr1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/340236/original/file-20200608-176550-1f5lfr1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/340236/original/file-20200608-176550-1f5lfr1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/340236/original/file-20200608-176550-1f5lfr1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/340236/original/file-20200608-176550-1f5lfr1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/340236/original/file-20200608-176550-1f5lfr1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">This large screen at the Russian Mission Control Centre for the Mir space station in Moscow shows the final orbits of Mir on March 23rd 2001, before it crashed into the ‘Oceanic Pole of Inaccessibility’ east of New Zealand.</span>
<span class="attribution"><span class="source">Author provided</span></span>
</figcaption>
</figure><img src="https://counter.theconversation.com/content/140066/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Steven Moore has previously received funding from NASA to conduct studies aboard the space shuttle and the Mir and International Space Stations. </span></em></p>The distance between the ISS and Earth is the same as about 3,850 football fields. To bring the station down, rockets will lower it a bit, and then gravity will send it crashing the rest of the way.Steven Moore, Professor/Deputy Dean Research, School of Engineering and Technology, CQUniversity AustraliaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1235062019-12-09T13:43:33Z2019-12-09T13:43:33ZWhat makes wine dry? It’s easy to taste, but much harder to measure<figure><img src="https://images.theconversation.com/files/303844/original/file-20191126-112517-1ctucpx.jpg?ixlib=rb-1.1.0&rect=786%2C0%2C2781%2C1752&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A lot of of chemistry and physics are behind how you perceive a sip of wine.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/pouring-wine-647742124">GANNA MARTYSHEVA/Shutterstock.com</a></span></figcaption></figure><p>When you take a sip of wine at a family meal or celebration, what do you notice?</p>
<p>First, you probably note the visual characteristics: the color is generally red, rosé or white. Next, you smell the aromatic compounds wafting up from your glass.</p>
<p>And then there’s the sensation in your mouth when you taste it. White wine and rosé are usually described as refreshing, because they have brisk acidity and little to moderate sweetness. Those <a href="https://www.winemag.com/2017/09/21/why-calling-a-wine-dry-or-sweet-can-be-simply-confusing/">low levels of sugar</a> may lead you to perceive these wines as “dry.”</p>
<p>People also describe wines as dry when alcohol levels are high, usually over about 13%, mostly because the ethanol leads to hot or burning sensations that <a href="https://doi.org/10.1021/acs.jafc.6b03767">cover up other sensations</a>, especially sweetness. People also perceive red wines as dry or astringent because they contain a class of molecules called polyphenols. </p>
<p><a href="https://www.scopus.com/authid/detail.uri?authorId=55360215200">As an enologist</a> – a wine scientist – I’m interested in how all the chemistry in a glass of wine adds up to this perception of dryness. People are good at evaluating a wine’s dryness with their senses. Can we eventually come up with a way to automatically assess this dryness or astringency without relying on human tasters?</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/303845/original/file-20191126-112522-dmwsr6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/303845/original/file-20191126-112522-dmwsr6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/303845/original/file-20191126-112522-dmwsr6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/303845/original/file-20191126-112522-dmwsr6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/303845/original/file-20191126-112522-dmwsr6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/303845/original/file-20191126-112522-dmwsr6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/303845/original/file-20191126-112522-dmwsr6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/303845/original/file-20191126-112522-dmwsr6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Molecules in grapes give them their various properties.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/ripe-red-wine-grape-ready-harvest-705572797">barmalini/Shutterstock.com</a></span>
</figcaption>
</figure>
<h2>The chemistry at the vineyard</h2>
<p>Everything starts with the grapes. If you taste a mature grape skin or seed at harvest, it will seem dry or astringent to you, thanks to a number of chemical compounds it contains.</p>
<p>Large molecules called condensed <a href="https://www.wineaustralia.com/getmedia/df422991-82ed-4125-b0f7-8395a63d438f/201005-tannin-management-in-the-vineyard.pdf">tannins</a> are mostly responsible for the astringency perception. These compounds are made up of varying types and numbers of <a href="https://doi.org/10.1021/bk-2002-0825.ch015">smaller chemical units called flavanols</a>. Tannins are in the same family of molecules, the polyphenols, that give grapes their red or black color. They tend to be larger in grape skins than in grape seeds, and consequently the skins tend to be more astringent, while the seeds are more bitter.</p>
<p><a href="https://doi.org/10.1021/bk-2002-0825.ch015">Grape varieties differ in how much</a> of each of these compounds they contain. In <em>Vitis vinifera</em> cultivars, like Pinot noir and Cabernet sauvignon, the tannin concentration varies from a relatively high 1 to 1.5 mg/berry. In cold-hardy hybrid grapes found in the Midwestern United States, <a href="https://doi.org/10.3390/fermentation3030047">like Frontenac and Marquette</a>, the concentrations are much lower, ranging from 0.3 to 0.7 mg/berry.</p>
<p><a href="https://www.wineaustralia.com/getmedia/df422991-82ed-4125-b0f7-8395a63d438f/201005-tannin-management-in-the-vineyard.pdf">Factors in the vineyard</a> – including site, soil qualities and amount of sun – affect the final concentration of tannins in the fruit.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/303614/original/file-20191126-84262-htad7o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/303614/original/file-20191126-84262-htad7o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/303614/original/file-20191126-84262-htad7o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/303614/original/file-20191126-84262-htad7o.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/303614/original/file-20191126-84262-htad7o.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/303614/original/file-20191126-84262-htad7o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/303614/original/file-20191126-84262-htad7o.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/303614/original/file-20191126-84262-htad7o.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Extracting tannins from red wines in the lab to characterize their chemical structure.</span>
<span class="attribution"><span class="source">Aude Watrelot</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>The chemistry in your mouth</h2>
<p>Basically, the more tannin there is in a wine, the more astringent it will be.</p>
<p>When you take a sip, the large tannin molecules <a href="https://doi.org/10.1016/j.tifs.2014.08.001">interact with proteins from your saliva</a>. They combine and form complexes, reducing the number of salivary proteins available to help lubricate your mouth. It leaves your mouth with a dry sensation – like if a snail were to lose its mucus layer, it would dry out.</p>
<p>Because everyone has a different composition and concentration of saliva proteins, and because the flow rate of saliva as you bring wine into your mouth varies, your perceptions of an astringent or dry wine won’t be the same as those of your friends or family. The alcohol level, pH and <a href="https://doi.org/10.1016/j.aca.2011.12.042">aroma of the wine</a> also influence how intensely and for how long you perceive a red wine’s dryness.</p>
<p>Since wine dryness is a perception, the most appropriate tool to appraise it is sensory evaluation. It requires panelists trained on the wine aroma, taste and mouthfeel based on prepared standards and other wines.</p>
<p>But winemakers would love to have a quick, simple way to objectively measure astringency without relying on human tasters. That way, they could easily compare this year’s wine to last year’s, or to another wine that is not available to be tested.</p>
<h2>Can we scientifically evaluate dryness?</h2>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/303846/original/file-20191126-112522-wmdoz4.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/303846/original/file-20191126-112522-wmdoz4.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/303846/original/file-20191126-112522-wmdoz4.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=787&fit=crop&dpr=1 600w, https://images.theconversation.com/files/303846/original/file-20191126-112522-wmdoz4.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=787&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/303846/original/file-20191126-112522-wmdoz4.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=787&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/303846/original/file-20191126-112522-wmdoz4.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=989&fit=crop&dpr=1 754w, https://images.theconversation.com/files/303846/original/file-20191126-112522-wmdoz4.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=989&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/303846/original/file-20191126-112522-wmdoz4.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=989&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Part of the apparatus the author and Tonya Kuhl used at UC Davis to measure the friction between two surfaces.</span>
<span class="attribution"><span class="source">Aude Watrelot</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>The challenge for me and my colleagues was to <a href="https://doi.org/10.1021/acs.jafc.9b01480">see if we could match up</a> the quantified chemical <a href="https://doi.org/10.1016/j.foodres.2018.09.043">and physical properties</a> in a wine to the trained panelists’ perceptions.</p>
<p>First, we used analytical methods to figure out the different sizes of tannins present in particular wines, and their concentrations. We investigated how these tannins interacted and formed complexes with standard salivary proteins.</p>
<p>My collaborators and I also used a physical approach, relying on a piece of equipment with two surfaces that are able to mimic and measure the forces of friction that occur in a drinker’s mouth between the tongue and the palate as wine and saliva interact. The friction forces increase between drier surfaces and decrease between more lubricated surfaces.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/304111/original/file-20191127-112484-xas3ab.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/304111/original/file-20191127-112484-xas3ab.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/304111/original/file-20191127-112484-xas3ab.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=630&fit=crop&dpr=1 600w, https://images.theconversation.com/files/304111/original/file-20191127-112484-xas3ab.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=630&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/304111/original/file-20191127-112484-xas3ab.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=630&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/304111/original/file-20191127-112484-xas3ab.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=791&fit=crop&dpr=1 754w, https://images.theconversation.com/files/304111/original/file-20191127-112484-xas3ab.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=791&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/304111/original/file-20191127-112484-xas3ab.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=791&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Researchers at Iowa State University’s Sensory Evaluation Lab passing wines to trained volunteers so they can report how dry they found particular wines.</span>
<span class="attribution"><span class="source">Aude Watrelot</span></span>
</figcaption>
</figure>
<p>Then, we trained human panelists to evaluate the intensity of dryness in the same wines and in a wine containing no tannins. </p>
<p>People perceived the wine containing the higher concentration of larger tannins as drier for a longer time than the wine without tannins. That made sense based on what we already knew about these compounds and how people sense them.</p>
<p>We were surprised, though, by our physical measurements in the lab, because they provided the opposite result as our human tasters’ perception. In the presence of too large or too many tannins in the wine, we recorded lower friction forces than in wines low in tannins. Based on the mechanical surfaces test, it seemed like there would be less dry mouthfeel than we’d expect in high-tannin wines. </p>
<p>My colleagues and I are planning to investigate this unexpected result in future research to improve our understanding of the dryness perception.</p>
<p>All its chemical and physical variables are part of what makes drinking wine a richly personal and ever-changing experience. Considering the impact of astringency on how individuals perceive a particular wine, a quick measure could be very helpful to winemakers as they do their work. So far, we haven’t been able to create a simple scale that will tell a winemaker that tannins at one certain level match up with a very particular dryness perception. But we enologists are still trying.</p>
<p>[ <em>You’re smart and curious about the world. So are The Conversation’s authors and editors.</em> <a href="https://theconversation.com/us/newsletters?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=youresmart">You can read us daily by subscribing to our newsletter</a>. ]</p><img src="https://counter.theconversation.com/content/123506/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Aude Watrelot has previously received funding from the American Vineyard Foundation.</span></em></p>Researchers would like to find a way to relate the human perception of dryness to the chemical and physical properties of the wine.Aude Watrelot, Assistant Professor of Enology, Iowa State UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1186412019-08-08T13:14:39Z2019-08-08T13:14:39ZNASCAR may be the fastest way to learn about physics<figure><img src="https://images.theconversation.com/files/286979/original/file-20190805-36399-kz4ouq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The laws of physics are on display at the Daytona International Speedway.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/february-26-2017-daytona-beach-florida-596617100">Action Sports Photography/Shutterstock.com</a></span></figcaption></figure><p>There’s just something thrilling about traveling at high speeds. Throughout history people have always pushed themselves to <a href="https://landspeedrecord.org/">go faster</a>, whether on foot, on horseback, on a boat or on a bicycle.</p>
<p>Nearly every weekend, today’s speed lovers can live vicariously by watching their favorite NASCAR drivers race around the track at death-defying speeds.</p>
<p>Maybe it’s the excitement in the crowd or maybe it’s the constant threat of danger that draws people to the sport. Or maybe its the feats of science and engineering that pull some spectators in. <a href="https://scholar.google.com/citations?user=w_InbNoAAAAJ&hl=en&oi=ao">As a physicist</a>, I love seeing all the physics principles on display during a NASCAR race. </p>
<h2>Speed</h2>
<p>NASCAR drivers travel at extremely high speeds, over 200 miles per hour. They accelerate so quickly that it takes them only around 3 to 3.5 seconds to go from zero to 60 mph. During this acceleration, the car must exert an average of 2,600 lbs of horizontal force against the track. This is comparable to the <a href="https://www.bio.fsu.edu/%7Egerick/bite_force/">bite force of a large American crocodile</a> or what it would take to lift a full-grown buffalo.</p>
<p>According to Einstein’s theory of special relativity, the faster you move through space, the slower your passage of time. So it’s fair to say that speed demon NASCAR drivers age a very tiny bit less than the rest of us. At the end of a 3.5 hour race, the drivers have aged about 0.5 nanoseconds less than the spectators who stayed still. If a driver raced nonstop at 200 mph for the next 50 years, he would age 70 microseconds less than the rest of us.</p>
<p>While NASCAR drivers are moving at incredibly fast speeds compared to the crowds in the stands, their speeds are small compared to what Einstein had in mind – like how fast light can travel, 670 million mph. The effect of relativity at the track is small, but it does exist.</p>
<h2>The track</h2>
<p>So how are drivers able to obtain these speeds?</p>
<p>As a car enters a turn, it naturally wants to continue in the direction it was originally going. To change direction to follow the curve of the oval-shaped track, a force must be applied.</p>
<p>The necessary force comes from the friction between the tires and the track. <a href="https://www.khanacademy.org/science/physics/forces-newtons-laws/inclined-planes-friction/a/what-is-friction">Friction</a> is the connection between the two that prevents them from sliding against one another. </p>
<p>So for drivers it’s a balancing act – they want to keep the pedal to the metal, but they can’t go so fast on a curve that their speed overpowers the maneuvering ability provided by friction. Go too quickly and the friction may not be enough to prevent the car from continuing in its original direction and sliding straight into the wall. Slow down too much and you fall behind the competition.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/286648/original/file-20190801-169684-hbw7kp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/286648/original/file-20190801-169684-hbw7kp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/286648/original/file-20190801-169684-hbw7kp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/286648/original/file-20190801-169684-hbw7kp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/286648/original/file-20190801-169684-hbw7kp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/286648/original/file-20190801-169684-hbw7kp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/286648/original/file-20190801-169684-hbw7kp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/286648/original/file-20190801-169684-hbw7kp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The banking of the track helps cars make those high-speed turns.</span>
<span class="attribution"><a class="source" href="https://unsplash.com/photos/Ur5VN_92g-k">Tim Trad/Unsplash</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>The way the track is designed can help out here. The turns are banked, meaning they are higher on the outside of the track and lower toward the center. Part of the force of the road pushing up on the car – what physicists call the <a href="https://www.khanacademy.org/science/physics/forces-newtons-laws/normal-contact-force/a/what-is-normal-force">normal force</a> – assists the frictional force of the tires and helps the car make it around the turn.</p>
<p><a href="https://www.nascar.com/gallery/talladega-superspeedway/#/0">Banking in the turns</a> at some of the fastest race tracks is comparable to the steepness of a playground slide. <a href="https://www.richmondraceway.com/About/About.aspx">Banking at Richmond International Raceway</a> allows cars to go approximately 1.3 times faster than they could without banking. Larger curves and higher banking, like those seen at Daytona and Talladega, allow the drivers to maintain a higher speed as they round those corners.</p>
<h2>Power</h2>
<p>Power is a measure of energy converted from one form to another in a set amount of time. In stock car racing, this conversion is from the chemical energy stored in gasoline to the kinetic energy of motion.</p>
<p>A NASCAR engine produces around <a href="https://www.nascar.com/news-media/2018/10/02/2019-rules-packages-announced-monster-energy-series/">750 horsepower</a> (560 kW), which exceeds a similar model street car that tops out around <a href="https://www.toyota.com/camry/features/interior/2550/2514/2532">300 horsepower</a>. During a race, the power conversion of a NASCAR engines is about 500 times the power usage of the <a href="https://www.eia.gov/tools/faqs/faq.php?id=97&t=3">typical American household</a> during the same period of time.</p>
<p>The cars’ power comes from burning gas as the engine rotates. The rotation of a NASCAR engine is 3.5 times faster than a standard street car and much more efficient, allowing it to combust more quickly and produce more power. </p>
<h2>Collisions</h2>
<p>With the high speed and power of stock cars come the risks of dangerous collisions. Some of the <a href="https://www.espn.com/racing/nascar/cup/news/story?id=5435268">hardest crashes in NASCAR</a> register around 80 G’s – that is, 80 times the acceleration of gravity that holds you to the planet. For perspective, amusement park rides top out around 6 G’s.</p>
<p>Safety elements try to extend the time, distance and area over which any collision takes place in an effort to lower these high forces. The principle is similar to the way gradually coming to a stop is less jarring than slamming on the brakes or the way a bed of nails spreads the weight of your body over a large area versus lying on a single nail. </p>
<p><a href="https://galvanizeit.org/project-gallery/nascar-safer-barrier">SAFER barriers</a> along the outside wall of the race track are made to crumple and dissipate a crash’s force over a large area. The front end of the car itself is also made to crumple, which extends the time of impact.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/286980/original/file-20190805-36381-naf1kw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/286980/original/file-20190805-36381-naf1kw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/286980/original/file-20190805-36381-naf1kw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/286980/original/file-20190805-36381-naf1kw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/286980/original/file-20190805-36381-naf1kw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/286980/original/file-20190805-36381-naf1kw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/286980/original/file-20190805-36381-naf1kw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/286980/original/file-20190805-36381-naf1kw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Safety elements inside a NASCAR vehicle go way beyond the seatbelt you have in your car.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/long-pond-pa-june-04-juan-55276939">Action Sports Photography/Shutterstock.com</a></span>
</figcaption>
</figure>
<p>Carbon fiber seats in the car absorb more impact energy compared to aluminum seats. They stabilize the driver by wrapping around the rib cage and shoulders, and spread the impact force over a larger area.</p>
<p>A 5-point harness connects the driver to the car, once again spreading the area of impact. It also attaches the driver to the car, so he or she slows with the crumpling car rather than continuing forward at full speed until impact. </p>
<p>So next time you head to the track or tune in on TV, ponder some of the physics of NASCAR, as well as the contributions of scientists and engineers working behind the scenes to improve the speed, power and safety of the sport.</p>
<hr>
<p><em>This article has been updated to correct the force on the track during a car’s acceleration.</em></p>
<p>[ <em><a href="https://theconversation.com/us/newsletters?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=thanksforreading">Thanks for reading! We can send you The Conversation’s stories every day in an informative email. Sign up today.</a></em> ]</p><img src="https://counter.theconversation.com/content/118641/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Christine Helms does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>High speeds, the threat of dangerous crashes, the excitement of the crowd – and the laws of physics on full display. A physicist explains the science of NASCAR.Christine Helms, Assistant Professor of Physics, University of RichmondLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1162252019-05-07T11:10:42Z2019-05-07T11:10:42ZLeonardo da Vinci’s early work on friction founded the modern science of tribology<figure><img src="https://images.theconversation.com/files/273022/original/file-20190507-103075-gzc660.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Leonardo da Vinci's experiments with friction underpinned the modern science of Tribology.</span> </figcaption></figure><p>For most people, the first thing that comes to mind when they think of Leonardo da Vinci is the Mona Lisa, or his sketches of the Vitruvian man. Fans of pulp fiction or popular cinema might even find their minds drifting to memories of The Da Vinci Code, the mystery thriller. Not me, though. I’m engaged in the study of friction, the force resisting the relative motion between surfaces. What most people may not realise is that Leonardo was responsible for the first systematic study of friction.</p>
<p>Friction is important to our daily existence and is present all around us. Starting with brushing teeth in the morning, driving a car and relying on a good grip of tyres, or looking at a mechanical watch with a confidence that all miniature parts are functioning properly to show us a correct time. As a field of science, study of friction is part of tribology, <a href="http://www.luuscimagazine.co.uk/?p=58">once called</a> “the most important subject no one has heard of”.</p>
<p>Tribology is the science and engineering of interacting surfaces in relative motion. It includes the study and application of the principles of friction, lubrication and wear. The word tribology derives from the Greek “tribo” (“I rub”) and the suffix “logos” (“study of”, “knowledge of”). The word is just over 50 years old and was <a href="https://search.proquest.com/openview/fb5d088988a2dc6ae28fe9d4d0f9015c/1?pq-origsite=gscholar&cbl=44644">coined by Peter Jost</a>, a British mechanical engineer considered the founder of the discipline. A <a href="https://www.machinerylubrication.com/Read/834/tribology-jost">report commissioned by the UK government</a>, highlighted the cost of friction, wear and corrosion to the country’s economy. </p>
<p>In 1966 that cost was estimated at 1.1%-1.4% of national GDP and, as a result, the UK government established several national centres to address problems associated with friction.</p>
<h2>Early works</h2>
<p>Leonardo understood very well that friction is a limiting factor in the design of his revolutionary machines. He worked on the subject for more than 20 years, as evidenced by his <a href="https://www.sciencedirect.com/science/article/abs/pii/S0043164816300588">beautifully illustrated notes and sketches</a>.
Leonardo distinguished between rolling and sliding friction and made observations that surface roughness has an impact on how easy it is to move different materials. His tribological experiments were dictated by his curiosity, but perhaps mainly by his pragmatic nature, as he needed reliable mechanical solutions for the design of his components. In his notebooks, we can find evidence of studying friction of simple blocks, but also screw threads, wheels and axes.</p>
<p>He was the first to record the laws of friction and he managed to achieve that by designing experiments using strings, pulleys and weights. Experiments based on the same principles are carried out on modern friction testers called <a href="https://www.tribonet.org/tribometer/">tribometers</a>. Some of those testers offer an ability to measure frictional forces at a tiny scale and might cost well above £200k due to their complexity and sophisticated nature of measurement electronics.</p>
<p>As it turns out, Leonardo never published his findings on friction – and he never got credit for his visionary findings. It was not until 1979, when the <a href="https://books.google.co.uk/books/about/History_of_Tribology.html?id=5GgfAQAAIAAJ&redir_esc=y">seminal book</a> on the history of tribology was published, and Leonardo’s now famous tribological sketches were revealed.</p>
<h2>Modern applications</h2>
<p>Today, tribologists around the world are building on foundations laid by Leonardo. <a href="https://doi.org/10.1007/s40544-017-0183-5">Modern studies</a> confirm the importance of urgent development of low friction surfaces to control the energy consumption, economic expenditure and CO₂ emissions on a global scale. Understanding the fundamentals of friction is key for a design of reliable and efficient electrical vehicles, wind turbines, and medical implants such as hip replacements.</p>
<figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/272300/original/file-20190502-103053-10069yn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/272300/original/file-20190502-103053-10069yn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=606&fit=crop&dpr=1 600w, https://images.theconversation.com/files/272300/original/file-20190502-103053-10069yn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=606&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/272300/original/file-20190502-103053-10069yn.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=606&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/272300/original/file-20190502-103053-10069yn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=762&fit=crop&dpr=1 754w, https://images.theconversation.com/files/272300/original/file-20190502-103053-10069yn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=762&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/272300/original/file-20190502-103053-10069yn.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=762&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">From humble beginnings: a modern versatile tribometer with high resolution of friction force measurement.</span>
<span class="attribution"><span class="source">Bruker</span></span>
</figcaption>
</figure>
<p>I have spent my career designing, testing and characterising surfaces by applying functional coatings, a field called “surface engineering”. My work involves using various tribometers and I find it extremely useful to reflect on my experiments and relate them back to early Leonardo’s studies.</p>
<p>Five centuries after Leonardo’s death and 50 years after tribology word was coined, tribologists are looking at a wider context of digital technologies and are designing functional surfaces that are sensing, responding and connected.</p><img src="https://counter.theconversation.com/content/116225/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Tomasz Liskiewicz receives funding from EPSRC</span></em></p>Once called ‘the most important subject no one has heard of’, tribology is now a key part of the fourth industrial revolution.Tomasz Liskiewicz, Head of Engineering, Manchester Metropolitan UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/466972015-08-26T15:55:35Z2015-08-26T15:55:35ZSolved: the mystery of why it’s impossible to pull apart interleaved phone books<figure><img src="https://images.theconversation.com/files/93053/original/image-20150826-15381-1auw98j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">No glue, only friction.</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/dannynic/3889396894/"> Danny Nicholson/Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span></figcaption></figure><p>People, trucks and even military tanks have tried and failed the task of pulling apart two phone books lying face up with their pages interleaved, like a shuffled deck of cards. While physicists have long known that this must be due to enormous frictional forces, exactly how these forces are generated has been an enigma – until now. </p>
<p>A team of physicists from France and Canada <a href="http://arxiv.org/abs/1508.03290">has discovered</a> that it is the layout of the books coupled with the act of pulling that is producing the force.</p>
<h2>The power of approximation</h2>
<p>Finding an approximate solution to a complex problem is an essential skill in science (and in life). Often we are faced with questions that we can’t answer exactly, but sometimes good enough is, well, good enough. <a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/1938/fermi-bio.html">Enrico Fermi</a>, one of the greatest physicists in the 20th century, has given his name to such “<a href="http://www.physics.uwo.ca/science_olympics/events/puzzles/fermi_questions.html">Fermi Questions</a>” – as he was famous for encouraging this skill in his students. </p>
<p>Here’s one example: “How many piano tuners are there in Chicago?”. I have no idea, and I’m not sure Fermi knew either. But by estimating the population of Chicago, the fraction that might play the piano, and how often a piano needs tuning, you can come up with a pretty good guess, without diving into the phone book (it’s probably closer to 100 than to 1,000).</p>
<p>Doing these “<a href="http://www.symmetrymagazine.org/article/june-2014/the-art-of-back-of-the-envelope-calculations">back-of-an-envelope</a>” calculations is usually the first step in approaching a scientific question. Sometimes that is as far as you need to go. Sometimes it tells us that the question is worth investigating more to find the exact answer.</p>
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<figcaption><span class="caption">Not even Brian Blessed can do it.</span></figcaption>
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<p>This is exactly what the team investigating the friction of phone books did. The back-of-the-envelope answer is friction between the pages. However, assuming the friction is proportional to the number of pages drastically underestimates the total force that is generated (which seems to rise exponentially with the number of pages). But previous attempts to improve this simple model – by including the effects of gravity and air pressure pushing the pages of the books together – have all failed to explain the result.</p>
<h2>Surprisingly simple</h2>
<p>So, when the back-of-the-envelope calculation fails, things get serious. In this case, the traction instrument was brought out (think the opposite of a vice), it was used to pull books apart while measuring the force required to do so. But not just any books. Rigorously prepared test books with specific numbers of pages, built from paper sheets of exact dimensions, interleaved to high precision.</p>
<p>Data in hand, a mathematical model was put together, and it turned out to be driven by a surprisingly simple fact. The pages of each book are separated by the interleaving and end up “spreading out”, lying at a slight angle from the spine. When the books are pulled away from each other, the pages want to move back closer together and end up squeezing the interleaved pages from the other book. And gripping something tightly greatly increases the friction.</p>
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<p>As an example, imagine a person with long hair in a swimming pool. While floating underwater, their hair can spread out – much like the pages of the books are spread out by the interleaving. Then, if our volunteer swims off, their hair will naturally move close together, following their head which is pulling it along. The pages of our books also want to move close together behind the thing pulling them (the spine of the book), but instead just squeeze more tightly on the pages of the other book, which are in the way. Pulling harder on the books only increases the friction.</p>
<p>This is an example of the geometrical amplification of friction, or how the layout of the books produces forces far beyond what is expected. Knots are another example, looping a rope around itself greatly increases the friction, resulting in a secure grip. The authors point out the recent resurgence of interest in this kind of problem and the general field of tribology, the study of <a href="http://www.imeche.org/knowledge/industries/tribology">surfaces in relative motion</a>. </p>
<p>This is being driven by the need to understand the structure and behaviour of new micro and nano-engineered materials, which have impact on many aspects of life from medical applications to solar cells. Interleaved carbon nano-tubes as the material of the future anyone?</p><img src="https://counter.theconversation.com/content/46697/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Gavin Hesketh receives funding from The Royal Society, the Institute for Particle Physics Phenomenology and the Science and Technology Facilities Council. He is affiliated with the Lightyear Foundation, a science education charity working in Ghana and the UK.</span></em></p>Take two phone books and lie them face up, with the spines facing away from each other. Then interleave the pages and try to pull the books apart. You will fail.Gavin Hesketh, Lecturer in Particle Physics, UCLLicensed as Creative Commons – attribution, no derivatives.