Rain on a Cart 2

Suppose rain falls vertically into an open cart rolling along a straight horizontal track with negligible friction. As a result of the accumulating water, the kinetic energy of the cart

A) Increases

B) Does not change

C) Decreases

D) Not enough information

Solution: C. There are no external horizontal forces, so the horizontal momentum of the cart before the interaction is the same as the momentum of the cart and water after the interaction. The momentum is unchanged but the mass increases. Using K = p^2/(2m), the kinetic energy decreases.