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Similar questions have been discussed that focused on coefficient of restitution. I recommend a madsci search for more information on collisions in general.
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using keywords: restitution and collision.
The difference is in the restoration force of the different materials. Rubber has more "bounce" because it is more elastic than leather. When the ball hits the ground and bounces back, the kinetic energy (motion) gets transfered to the surface of the ball when it is stored (briefly) as potential energy, then the energy is returned to kinetic energy when the ball bounces back. Some of this potential energy is lost (it gets turned to heat energy due to frictional effects in the material) so the ball does not bounce up with the same kinetic energy with which it hit the ground.
The leather ball surface does not have as strong a restoring force as the rubber ball surface. Beyond a certain deformation a ball surface will begin to lose energy very quickly. This is why air is put in the ball in the first place. The leather ball will enter this critical range of energy return for lower energy than the rubber ball. So for a given amount of kinetic energy (1/2 * mass * speed ^2), the leather ball and rubber ball might deform roughly the same on impact with the ground, but the rubber does a better job of holding the energy at deformation and returns a greater portion back to kinetic energy.
These sorts of collisions are seen all the time in all sorts of sports. For example, a golf ball struck by a golf club. A couple answers about deformation of the golf clubhead and ball and energy transfer have been posted as well.
Sincerely,
Tom "Dead Ball" Cull
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