Throwing Balls 4

You are initially at rest on a cart which has a solid ceiling, solid floor, and solid walls on all sides except for the right side which has a small aperture. You throw balls at random and some of them eventually exit the cart. Is the cart put in motion?

a) Yes, it moves to the right

b) Yes, it moves to the left

c) No, it remains in place

d) Depends

Solution: B. There are no horizontal external forces on the system of cart, balls, and person. The initial momentum is zero (everything at rest), so the final total momentum must be zero by conservation of momentum. The balls inside the cart will have a total momentum of roughly zero because their velocities are random. The balls that exit the cart will have momentum to the right, so the cart must move to the left to have total momentum of zero for the system.