The Physics of a Rolling Rubber Band 226
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)."
Brakes, please. Please? (Score:5, Informative)
Direct link to the .FLV (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 [aaas.org] (320x240, 17 seconds, 1.1MB)
Re:BZZT! WRONG! (Score:3, 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 ;-)
Re:Physics... (Score:1, Informative)
But all cars get shorter as they get faster - it's the Lorentz-Fitzgerald Contraction (http://en.wikipedia.org/wiki/Length_contraction), due to approaching the speed of light.