# Stacked Rubber Balls

A basketball and a small ball are stacked and dropped simultaneously. Upon collision with the ground, energy from the bottom ball transfers to the top one and it bounces higher than it would alone.

Equipment:

• Smaller rubber ball

Demo:

1. Drop each ball from the same height individually. Notice how far each ball bounces back.
2. Hold the rubber ball on top of the rubber ball and release them at the same time.

Explanation:

There are two main effects that are occurring when you drop these two balls. One is a transfer of momentum and the other is a transfer of elastic potential energy to kinetic energy.

1. Momentum Transfer: Right before the basketball hits the ground it has a momentum of $m_{1}v_{1}$, where $m_{1}$ is the mass of the basketball and $v_{1}$ is its velocity. Theoretically if the basketball has a perfectly elastic collision with the ground we just have to worry about the collision with the rubber ball on top. The rubber ball has a momentum of $m_{2}v_{2}$. If the basketball and the rubber ball have a perfectly elastic collision. Since the basketball has a much larger mass it therefore has a much greater momentum. This momentum transfer is then given by the equation below:

$m_{1}v_{1}+m_{2}v_{2}=m_{2}(v_{1}+v_{2})$

Notes:

• Make sure the balls are fully inflated (use ball pump in drawer)