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Elastic vs inelastic collisions

  1. Sep 12, 2014 #1
    ok, so as i understand an elastic collision conserves energy, and an inelastic collision doesnt conserve energy.
    also that the energy loss in inelastic collisions is due to the fact that energy is required to "squash" or deform the object.

    ok, maybe its just the terminology(it probably is), but im completely confused here. if i throw a rubber bouncy ball at the ground (inelastic?), it will bounce higher than i threw it, in fact ill probably lose it and never find it again. If on the other hand i throw a ball bearing(elastic?) at the ground it might ricochet a little but in general its not going to go very far.
    Now im thinking the rubber ball must be elastic, and the ball bearing is inelastic,(which is lexically correct, although maybe not scientifically) but i also read that the collisions between billiard balls is elastic.

    do bouncy balls somehow violate the laws of physics to attain a higher energy after a collision? what is this sorcery?
  2. jcsd
  3. Sep 12, 2014 #2


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    If you just drop a rubber ball onto the ground, it won't bounce higher than where you dropped it from. The reduction in height is a measure of the amount of kinetic energy converted into a different form when it bounced.

    If you throw the ball downwards, you are giving it the same downward speed as if you had dropped it from a higher point, and it bounces to (nearly) the same height you would have dropped it from.

    Bouncing something on the ground also depends on the "elasticity" of the ground. Suppose you drop a rubber ball and a steel ball with the same mass, both from the same height. They hit the ground at the same speed, so the amount of "impulse" (change in momentum) in the two impacts is the same. But impulse = force x time, and the squashy rubber ball is in contact with the ground for a much longer time than the steel ball. So the average force on the ground is smaller for the rubber ball, and that is less likely to "waste" energy by making a permanent dent in the ground, or whatever.

    If you bounce a steel ball on another steel ball, you find they are almost perfectly elastic. You can see that if you play with a "Newton's cradle" - or find a video on Youtube. The only "special" things about the Newton's cradle are first that all the balls have the same mass, and second it is designed so they bounce "accurately" off each other moving in a controlled direction.
  4. Sep 13, 2014 #3


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    Staff: Mentor

    In everyday use, the word "elastic" is used to mean "stretchy". Forget that usage, in physics.

    A quartz marble is almost perfectly elastic, in a collision. But not very stretchy, is it!
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