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Suppose there are two balls, one of rubber and the other metallic. There are of the same mass and are thrown on a wall with the same velocity. Why does a rubber ball bounce back while a metallic ball simply falls down after striking with the wall? I know it has got to do something with the change in linear momentum and its elasticity but what?

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3 Answers 3

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If you use a plate glass window instead of a wall you'll find that the rubber and iron balls bounce by a similar amount (though be careful throwing iron balls at windows :-).

It's a basic principle in physics that energy cannot be lost. The rubber ball starts off with kinetic energy, hits the wall, and rebounds moving with about the same kinetic energy. So no energy is lost. If the iron ball doesn't bounce it must mean that the energy it originally had has been transferred to the wall.

Rubber balls are soft, so they decelerate relatively slowly and they deform and spread out as they hit the wall. This means that the pressure they exert on the wall while they are bouncing is relatively low. By contrast an iron ball is very hard so it stops very suddenly and all the force it exerts on the wall is concentrated on a small area. That means the pressure is high enough to damage the wall. It might cause a visible dent, or it might just cause cracks within the wall that you can't see. In both cases energy is used in damaging the wall, and this energy comes from the motion of the ball. That means little energy is left for the iron ball to bounce back.

I started by saying the iron ball would bounce off plate glass. This is because plate glass is very rigid and provided you don't shatter it the glass is not damaged by the iron ball. Since no energy is absorbed by the glass, the iron ball bounces back just as the rubber ball does.

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okay, what if it was a metallic ball instead of an iron one? –  shaurya gupta Oct 30 '13 at 11:44
My book says that this happens because the rubber ball suffers a greater change in linear momentum. –  shaurya gupta Oct 30 '13 at 11:57
@shauryagupta: the important property is the rigidity and density of the ball, so steel, nickel or and metal would be fairly similar to iron. Glass or hard plastic balls have a similar rigidity but weigh less so they would damage the wall less and bounce more. The linear momentum is just the mass of the ball times it's velocity. The final momentum is the result of the physics of the bounce not a cause of it. –  John Rennie Oct 30 '13 at 12:02
@shauryagupta Your book is either wrong or you are reading it incorrectly. The rubber ball suffers a greater change in linear momentum because its final velocity is greater than that of the iron ball and because this velocity is an opposite direction (assuming no lateral components). It is not the other way round. –  Apoorv Khurasia Oct 30 '13 at 12:27
The YouTube page says it's an animation. Now we can prove all the Roadrunner-Coyote physics! –  DJohnM Oct 30 '13 at 20:04

Because you need for "bounce back" to have a reversible deformation of both the ball and the wall.

For the rubber ball, the ball gets elastically deformed and the wall not ; this is all reversible.

For the metal ball, the ball gets mostly not deformed but the wall gets somehow scratched because of the more localized impact (resulting from the ball not willing to change shape) ; this eats up the kinetic energy of the ball.

Use an iron ball on an iron wall, you'll notice near-perfect bounce !

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I just used but considering that the velocity is low enough for the rubber ball to bounce but not for the metallic ball... –  shaurya gupta Oct 30 '13 at 11:41
The velocity is not so much in question as the ability of the wall to deform and bounce back while giving all the initial energy back –  Nicolas Oct 30 '13 at 12:24
what if the wall does not get scratched at all?? –  shaurya gupta Oct 30 '13 at 12:27
Well in that case it means that either the ball itself gets scratched, or it bounces back. Note however that even very small scratches or bumps amount to significant energy loss. –  Nicolas Oct 30 '13 at 15:46
But how would you compare the bounce of rubber ball and the metallic ball considering the momentum before the striking is same... –  shaurya gupta Oct 30 '13 at 15:49

Because a rubber (or as in this image a golf ball) can deform without breaking apart. The bigger deformation stores more energy in the rubber ball than the iron one.

Microscopically, the rubber ball is made from a polymer and they're very plastic (elastic/deformable) in comparison with the iron ball which has a rather homogeneous crystalline structure (highly ordered) which translates into a low elasticity.

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And how does that elasticity help in bouncing? –  shaurya gupta Oct 30 '13 at 11:40
In being able to deform and store the energy and then to push back from the surface (change in momentum implying a force applied to the ball) to bounce. –  Ignacio Vergara Kausel Oct 30 '13 at 11:55

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