bounce collision

Question

A tennis ball that has a mass of 0.05 kg is rolling east with a velocity of 10 m/s collides with a baseball that has a mass of 0.1 kg that is rolling west with a velocity of 15 m/s. After the collision, the tennis ball is rolling to the west with a velocity of 12m/s. Assuming the collision is perfectly elastic and friction between both balls and the ground is negligible, what is the velocity (magnitude and direction) of the baseball after the collision?

Answer 1

Momentum is mass x velocity, and is a conserved quantity.

The tennis ball therefore has a momentum of 0.5 kg·m/s east, and the baseball has a momentum of 1.5 kg·m/s west. The total momentum of the system is their sum 1 kg·m/s west.

After collision, the tennis ball is rolling west with momentum 0.6 kg·m/s west, so the remaining momentum must belong to the baseball.

The baseball's momentum is then 0.4 kg·m/s west, and dividing by its mass gives us a velocity of 4 m/s west.