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The Physics of a Rolling Rubber Band

Posted by CmdrTaco on from the thank-god-that-is-solved dept.

sciencehabit writes "Modern physics can get complicated. Sure, researchers know exactly what forces act on a ball rolling down an incline — an experiment that helped Galileo develop universal laws for movement and acceleration. But what happens when a deformable shape like a rubber band rolls around? A new study reveals that the faster it goes, the more squashed it gets (video included)."

31 of 226 comments (clear)

  1. Physics... by Pojut · · Score: 3, Insightful

    ...is mind-boggingly awesome. I can't understand the math at all, but I understand the way things generally act. So cool (and so insanely complicated! Think about something like a key being inserted into a lock...and that's just simple, everyday stuff!)

    --
    Living With a Nerd
    1. Re:Physics... by SpinningCone · · Score: 4, Interesting

      i agree. I always liked physics made the world look different (like "car breaks are kinetic to thermal energy converters"). never could really get into dynamics though. i remember my teacher describing the the problem of rotational inertia of a deformable object (like a jelly disk) faster you spin the more it changes shape which changes its inertia.

      props to the people out there with the knack and persistence to solve crap like that.

    2. Re:Physics... by Anonymusing · · Score: 4, Funny

      Physics is pretty cool.

      I liked this quote from the article:

      As far as the potential applications, Clanet waxes futuristic. "I can imagine [designing] a car. The faster it goes, the more it deforms and the less friction it has with surrounding air, so it can go even faster. It would be a fantastic car."

      A car that changes its shape as it drives? Getting shorter, even? "Ouch, slow down, you hit my head!"

      Automobile safety experts would have a field day with that.

      --
      Liberal? Conservative? Compare perspectives at Left-Right
    3. Re:Physics... by Seismologist · · Score: 2, Interesting

      ...is mind-boggingly awesome.

      Actually you can often make a simple assumption and work off of F = m*a or some other well established theorem...

      As for the math, now that is some pretty mind boggling stuff. Some of the math that was used to pull string theory together is pretty bleeding edge on top of the physics part of it. PBS had a interesting show on string theory(you can watch in three installments on PBS). What struck me the most was how splintered the physics community was as many researches were doing the math a certain way different from each other, but it was found to be all the same by another physics/math guru when he proposed 11 dimensions instead of 9 like the other researches had inferred.

      --
      ~ In Trust, We Trust ~
    4. Re:Physics... by EdZ · · Score: 3, Insightful

      Because air behaves differently at different speeds. Once you got fast enough, shockwaves become the limiting factor rather than fluid fraction. Then you have cavitation, and things like compression heating. What is most efficient at one speed is not most efficient at all speeds.

    5. Re:Physics... by VolciMaster · · Score: 3, Funny

      I love Ben Rich's quote on that: "no one's been wing-walking at Mach 3 to verify that assumption" :)

      --
      antipaucity
    6. Re:Physics... by VolciMaster · · Score: 2, Insightful

      Yeah, that quote bothered me too - I'm thinking, why not just design it with the least friction with the air to start with - why have it be less efficient at slow speeds in other words?

      Unless Clanet was referring to the design process itself - use an elastic model in a wind tunnel (or simulate the whole thing) and observe it's deformation to determine the shape with the least friction with the air (or call it coefficient of drag, like everyone else does :-)

      At first I was thinking it might have been typical media-distorted science, but when they threw in that quote from Clanet, it seemed more that the science is hard to take seriously too.

      Just a quick thought, but at low speeds aerodynamic efficiency is of very low impact (eg a barge at 2 kts and a kayak at 2 kts). The faster they go, the more that efficiency matters - having a material that could deform to improve flow as speeds increased could be a good thing - especially if it were used around the freight compartments of a tractor-trailer or rail car: squishing-down to more evenly flow around the carried contents could have some promise.

      --
      antipaucity
    7. Re:Physics... by Thing+1 · · Score: 2, Interesting

      The other thing you can do is add texture which separates the flow from the object and usually reduces the size of the low pressure bubble behind it, and this is why golf balls are stippled.

      I remember a MythBusters episode where they added a bunch of clay to a car (adding quite a bit of body weight), and then carved out dimples like a golf ball would have.

      The car ended up getting better fuel efficiency than the original (lighter, smoother) car. I was really impressed, especially because it had to overcome the added weight.

      --
      I feel fantastic, and I'm still alive.
  2. Delight to read... by ZeroExistenZ · · Score: 2, Funny

    But what happens when a deformable shape like a rubber band rolls around?

    ... the article sounds like the things I used to wonder about and do during boring classes in highschool.

    --
    I think we can keep recursing like this until someone returns 1
    1. Re:Delight to read... by jamesh · · Score: 2, Funny

      ... the article sounds like the things I used to wonder about and do during boring classes in highschool.

      Same here, except I was more like "I wonder if I can hit that kids sticking-out ears with a rubber band from here", without thinking through what would happen if i _did_ hit them (which should have been obvious in retrospect... it was for that reason I sat at the back).

    2. Re:Delight to read... by Spad · · Score: 4, Funny

      I like to imagine that all scientists operate on this principle. They sit around doing boring paperwork until one of them says "I wonder what happens when a deformable shape like a rubber band rolls around?", to which one of the others replies "Quickly, to the lab!" and they all run off to investigate it.

    3. Re:Delight to read... by Scatterplot · · Score: 2, Funny

      No, it depends on how fast he was rolling. I think.

  3. Wow, interesting! by rotide · · Score: 4, Interesting

    If you would have asked me how it would react as it rolled faster and faster, I would have just assumed it would have gotten "rounder" and possibly larger (elastic) due to centrifugal force.

    Always amazes me how things don't always work as expected. Nature, physics, etc, are truly interesting... no, fascinating. Now if only I had a better grasp of higher level maths and wasn't a Network Engineer (data plumber).

    1. Re:Wow, interesting! by Spad · · Score: 3, Funny

      ...due to centrifugal force.

      My high school physics teacher used to electrocute (With a handheld generator made from a rotary pencil sharpener) people for saying that; also for misspelling accelerate or satellite.

    2. Re:Wow, interesting! by ledow · · Score: 4, Interesting

      Yes, but then people "weigh" themselves in kg's by standing on a scale that is affected by gravity.

      There are certain things that, although "visible" by ordinary people and named, don't actually exist or exist only because we *perceive* them to exist, like that optical illusion with the white triangle that isn't ACTUALLY there.

      Centrifugal force may be misnamed (i.e. not a force), it may be incorrect, but it's generally accepted that "a force" exists that has an effect on your when you're spun in a circle. Just because the direction / origin / name of that force is incorrect is no reason to tell people that they're stupid for having felt it and knowing what it is before you explain its origins.

      Back in the 60's there was an advertising campaign by scientists working on the behalf of government to target heat loss in elderly people's properties. It encouraged old people to "keep the heat in". It didn't go down well and it took them years to discover why. Eventually it was changed to "keep the cold out" and more elderly people understood that. "Cold" doesn't actually exist, it's just the absence of heat, but old people didn't think that way as easily (and who can blame them? "Shut the door, you're letting the cold in" is a common cry in my family - despite the fact that you're neither letting cold in nor arranging for some mystical "cold" entity to enter your property rather than, say, air with slightly less heat).

      There's 100% pedantic accuracy. There's complete bollocks. And somewhere in the middle is how *everybody* thinks, even if they know both extremes in detail.

    3. Re:Wow, interesting! by electrostatic · · Score: 2, Interesting

      "Spinning faster = more velocity perpendicular to slope on the leading edge of the loop. It makes sense that it would flatten out."

      Very good point. The back edge of the loop is being accelerated perpendicularly upward. IOW, the small length of rubber that's breaking contact with the table is yanked -- accelerated -- upward to a high vertical velocity. Therefore, it will rise higher that it does at a slower rolling speed. Like throwing a ball upward with a high velocity against gravity, it reaches a higher distance. In the case of the rubber loop the restoring force is tension in the rubber just ahead of the peak.

      But there's that difficult-to-model problem of elasticity. I imagine the fast-rising rubber is pulled downward by tension in the bit of rubber just ahead of the highest point. Most of its upward momentum is opposed by the force of that stretched bit. And the kinetic energy, which is proportional to the square of the velocity, is transferred to mechanical energy (force X distance) stored in the stretched bit. Since KE is non-linear (square of velocity) you often get unexpected behavior. Here we get into differential equations!

      Regarding the role of elasticity in the transfer of momentum and kinetic energy in the shaping of the rolling loop, it would be interesting to do an experiment using a non-elastic loop. An example would be a metallic chain. I expect the shape would be different.

    4. Re:Wow, interesting! by Chris+Burke · · Score: 2, Insightful

      Yeah, stupid meteorologists, talking about "cold fronts". No such thing! They should say "Fronts containing less heat." But then again all fronts contain heat, so what's a warm front? It's just warm compared to cold fronts.

      Maybe it's because of my electrical engineering schooling and years spent acting as if it was the positive charge carriers that were moving, but I don't see any problem with saying "let the cold in". Cold is a negative heat delta. Big whoop. When you open the door in the winter, cold air blasts you in the face, cold air got in the house, and the house becomes colder. You let the cold in.

      "Cold" may not exist but the concept is valid, and you can only tell the difference because of absolute zero. It's very similar to negative energy, a common concept in physics. The question of whether or not it "exists" vs is a valid way to conceptualize energies that only average to zero, is just how you view it.

      There are legitimate contexts where it is 100% pedantically accurate to talk about things that don't exist.

      --

      The enemies of Democracy are
  4. Brakes, please. Please? by Kupfernigk · · Score: 4, Informative

    I'm sorry but this is such a common mis-spelling on Slashdot that it's getting to me. Cars have brakes. "Car breaks" means it stops working because of mechanical or electrical failure. Spellcheckers can't fix homophones.

    --
    From scarped cliff or quarried stone she cries "A thousand types are gone, I care for nothing, no not one."
    1. Re:Brakes, please. Please? by Anonymous Coward · · Score: 4, Funny

      Spellcheckers can't fix homophones.

      I can tell you have your spellchecker on, though I think you ment to write "homophobes".

      I too believe they should be fixed.

    2. Re:Brakes, please. Please? by Bromskloss · · Score: 3, Funny

      Cars have brakes. "Car breaks" means it stops working because of mechanical or electrical failure.

      I honestly thought he was talking about car crashes and even though that was a strange way of saying it, I convinced myselft that is was physically sound.

      --
      Swedish plasma phys. PhD student; MSc EE; knows maths, programming, electronics; finance interest; seeks opportunities
    3. Re:Brakes, please. Please? by Anonymous Coward · · Score: 2, Funny

      Tell that to two Slashdot users whose who's are wrong too. They're messing up their you'res while your there's are perfect.

    4. Re:Brakes, please. Please? by boxwood · · Score: 3, Funny

      a bit of tape stuck over the bottom left corner can fix a homophone.

    5. Re:Brakes, please. Please? by PiSkyHi · · Score: 2, Funny

      I think the key to this is that when a car brayx, it slows down and will eventually stop moving.

  5. Direct link to the .FLV by Anonymous Coward · · Score: 3, Informative

    For those of us where the player won't launch when you click "play video" in the article, here's a direct link to the flash video:

    http://sciencevideo.aaas.org/sciencenow/snow_ribbon_250.flv (320x240, 17 seconds, 1.1MB)

  6. This requires Yakety Sax by cangrande · · Score: 5, Funny

    All science videos are improved by Yakety Sax.

  7. Miracle by JustOK · · Score: 5, Funny

    Fucking rubber bands, how do they work?

    --
    rewriting history since 2109
  8. Flying out of the drum by azmodean+1 · · Score: 3, Interesting

    I was amused by this aside:

    (The team couldn't study what happened when the two sides touched: The friction of the two sides moving in different directions sent the rubber bands flying out of the drum.)

    What? It seems pretty obvious that they could see exactly what happened when the two sides touched, "The friction of the two sides moving in different directions sent the rubber bands flying out of the drum".

  9. BZZT! WRONG! by gbutler69 · · Score: 2, Interesting
    You, like many of the others above, FAIL! There is Centrifugal Force. If I spin a rock at the end of a rope, the rock experiences a "Centripetal Force" which is a force pulling towards the center of a circle, where my hand is, that is perpendicular to it instantaneous velocity. Note, it is not only experiencing a "Centripetal Force", otherwise it would just accelerate along the radius toward the center. It is in fact also experiencing a force along the tangent (i.e. the diretion of instantaneous velocity) of the same circle. But, here is where it gets interesting. It is actually experiencing neither of those. It is in fact experiencing a force that is at an angle slightly between the direction of the tangent to the circle and along the radius. We can break that actual force (the real direction I'm actually pulling on the string) into the component along the radius and the component along the tangent, but, there really isn't two separate forces acting, just one. So, what you are calling "Centripetal Force" doesn't actually exist either. It's just a a convenient name for the component of the force pulling inward along the radius. Now, according to Newton's Laws, for every force (action) there is an equal an opposite force (re-action). So, if there is a corresponding force pulling inward on the rock (the "Centripetal Force") then there is also a force pulling outward along the radius (the "Centrifugal Force"). In fact, the Rock is pulling on my hand with such an equal and opposite force. So, my hand (and the string) is experiencing "Centrifugal Force". So, "Centrifugal Force DO EXIST!" You FAIL! Go back and re-read your Physics text-book and try again!

    Thank You for Playing!

    --
    Over-the-top Response Guy! Giving "Over-the-Top Responses" since 1970.
    1. Re:BZZT! WRONG! by gbutler69 · · Score: 2

      That force is directed outward radially. It is a centripetal^h^h^h^h^h^h^h^h^h^h^hCENTRIFUGAL force. Yes, I have calculated this using Calculus as well. Yes, I know there is not CENTRIFUGAL FORCE acting on the ROCK, but, there is a CENTRIPETAL^h^h^h^h^h^h^h^h^h^h^hCENTRIFUGAL FORCE acting on my hand and the string that IS a manifestation of the intertia of the rock.

      Woops! In my haste I had a couple of typos.

      --
      Over-the-top Response Guy! Giving "Over-the-Top Responses" since 1970.
    2. Re:BZZT! WRONG! by XSpud · · Score: 2, Informative

      Yes you will have burned calories but you will have done no work on the wall. The work performed will have been within your body caused by the contraction of muscles. The energy expended will have been converted to heat, presumably now lost to the environment through your skin if you're still alive ;-)

      If you want to consider all forces and energy transfers within a human body when pushing against a wall, there's a lot to consider - it's far simpler to assume the body is rigid and has no internal structure that you need to be concerned about - in your example perhaps think of a rigid body leaning against a wall, which will impart a force but does not expend energy. In this case we would only need to consider mechanical properties of the system rather than the thermodynamic properties that apply to all real-world systems.

      Of course if you are a biologist it might be valid to look at the the energy expended pushing against a stationary wall for an hour. In this case they might want to consider the the source of the energy (chemical), how forces act on the skeleton, the mass of limbs etc - not very useful though if you want to understand fundamental concepts of physics.

      Reading through the discussion above and below, it's clear that posters are talking about different systems - this is why the first thing an applied mathematician or physicist does is to draw a diagram, and to state any assumptions. For example I think gbutler69 was talking about a system with the "hand" moving in a circle to impart a force to maintain the kinetic energy of the rock, where KE was being lost to air friction, and the responders were assuming a frictionless system with a rigid, fixed "hand".

      To prevent a similar flamewar I should mention that in my example above, the rigid post is attached to the ground, which is also rigid and has infinite mass ;-)

  10. Obvious by wcrowe · · Score: 3, Funny

    A new study reveals that the faster it goes, the more squashed it gets.

    Well duh. Of course it does. Anyone who has watched a Roadrunner cartoon knows that.

    --
    Proverbs 21:19