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Why things bounce

  1. Oct 4, 2004 #1


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    this is a rather stupid question: the other day, I found a bouncing ball in my room, and I started boucing it around. And I want to know what makes it bouce?
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  3. Oct 4, 2004 #2


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    When the ball hits a hard surface it deforms. The deformation is a form of potential energy. When the ball restores its shape, the energy stored in the deformation is converted back to kinetic energy.
  4. Oct 4, 2004 #3


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    Mostly, it has to do with the elastic properties of the materials that make up the two colliding objects. And far from being a stupid question, it's in fact quite a hard question to answer simply.

    The property that describes the bounciness between a pair of objects is known as the coefficient of restitution. A very bouncy ball on a rigid floor will have a coefficient of restitution close to 1. On the other hand, a lump of putty, will have a coefficient of restitution that is 0.

    What this coefficient measures is the elasticity of the collision, or how much of the kinetic energy remains in that form.

    Addressing the converse question is instructive. What causes something to not bounce ? When a ball falls from a height, you would expect it to bounce back to that same height because of energy conservation. What happens during a bounce ?

    The ball has kinetic energy just before it touches the floor. Now as it gets squeezed against the floor (imagine all this happening in real slow motin), this kinetic energy is converted into the potential energy of the elastic material that makes up the ball (this is just like the potential energy contained in a compressed spring). Now if all the potential energy got converted back into kinetic energy (a compressed spring will not stay compressed if let be, it will spring back), the ball would bounce back up to the original height.

    However, there's always some fraction of the energy that gets absorbed by the ball (causing a small amount of permanent deformation), or gets released as sound or heat. The conversion of elastic potential energy to sound or heat depends on the nature of binding forces between the atoms/molecules in the ball.

    It is this conversion of kinetic energy to other forms of energy, that causes the ball to lose height after bouncing. The more elastic the material of the ball, the less will be the conversion to other forms of energy and hence the bouncier will be the ball.
  5. Oct 4, 2004 #4
    bouncing balls

    You may see it all as a game of energy.

    The ball will always want to have minimum potential energy. So, when you release it from your hand, it "sees" a decrease in potenitial energy in going down. So it does that.

    But when it lands, there is an increase in potenitial energy due to compression. So it must go up to decrease that. But, in going up to decrease that, there is again an increase in the potential energy. So, what does it do know? It makes an "optimization" and goes up, not necessarily to the height from where it was released but depending upon the amount of compression and the related potenitial energy.
    The process is repeated again and again.

  6. Oct 4, 2004 #5


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    And if you're asking "what is the nature of elasticity?" the answer is that it is due to the electromagnetic attraction between atoms or molecules in the substance. Rubber is a polymer and is composed of many chains of atoms all linked together through bonds. When the chains are stretched, the atoms are pulled in opposite directions, and the bonds store the potential energy like a microscopic spring.

    - Warren
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