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So just some background for the question, if two pucks with velcro on each side on very slippery ice with close to no friction collide, they stick together. Thus, the collision is inelastic.

However, I'm confused as to where the lost kinetic energy goes? People have said sound and thermal energy (from friction), but since there are velcro pieces on each side of both pucks I'm wondering if some of the energy is stored in the velcro as elastic potential energy?

Basically, I'm not sure what to call the energy stored in the velcro (if any is even stored in the velcro) because it's not exactly deformed from the collision, it just sticks with the other velcro piece.

Anyway if someone could clarify 1. Where the kinetic energy goes and 2. If it does go to the velcro, what kind of energy is it and how does it "affect" the velcro, that would be very much appreciated :)

("affect" being like how kinetic energy increases the speed of an object, or thermal energy raises its temperature, or how elastic potential energy in a spring compresses or stretches it).

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  • $\begingroup$ Various part of the velcro pads will end in a strained configuration, but it's disordered and I presume relatively little energy in the grand scheme of things. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Apr 8, 2018 at 21:52
  • $\begingroup$ I wonder if energy is stored by the fact that the velcro is sticking together? $\endgroup$
    – user191799
    Commented Apr 8, 2018 at 21:56
  • $\begingroup$ Energy can be stored via the elastic strain in an object (as mediated by its stiffness), but I don't really see how the hook-and-loop closures in velcro after a collision are deformed in any way that's significantly different from how they were deformed before the collision. $\endgroup$
    – Chemomechanics
    Commented Apr 8, 2018 at 23:01

2 Answers 2

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As you can see in this video, velcro generally works because lots of little hooks (the "rough" side) become tangled in a web of little hairy loops (the "fuzzy" side). When the pucks collide, much of the energy goes to deforming the hooks and hairs as they are smashed together. Similar to how bending a paperclip repeatedly causes noticeable heat to be created, bending tons of little hairs and hooks generates a tiny bit of heat in each little bit.

As the collision occurs, some of the energy is dissipated as heat as demonstrated above, while other energy may be dissipated as sound (an acoustic wave through the air). That said, intuitively I don't reckon smashing velcro-coated pucks together would generate much sound. If there is still energy left over though, the pucks will start to separate again. They won't get very far though, because (if the velcro is strong enough), the hooks will catch on the hairs, and prevent bulk motion. This process will lead to more heat being dissipated as the hooks and hairs uncoil.

This "ringdown" process continues until all the energy has been dissipated as heat. (This process is sort of similar to friction because it involves a lot of small, but macroscopic things rubbing against each other. If you'd like to call it friction, I say go ahead, but know that to actually calculate a friction force directly (as is done in introductory physics courses) would be very challenging as you likely have thousands of rubbing interfaces!

Finally, I do not think that very much energy would be converted to potential energy. Once the pucks have come to rest, there is no way you could get them to spontaneously separate. You can't recapture any of the energy "lost" in this collision.

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The kinetic energy lost turns into potential energy that will be released when the Velcro becomes undone.

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