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 speed of the cart
A) Increases
B) Does not change
C) Decreases
D) Not enough information
Solution: C. The cart and rain have no external horizontal forces, so total horizontal momentum remains unchanged. The water now has some horizontal momentum as it moves with the cart, so the momentum (and speed) of the cart must decrease.