Question
A cart full of water travels horizontally on a frictionless track with initial velocity v. As shown in the diagram, in the back wall of the cart there is a small opening near the bottom of the wall that allows water to stream out. Considering just the cart itself (and not the water inside it), which of the following most accurately describes the characteristics of the cart?
Speed Kinetic Energy
(A) stays the same stays the same
(B) increases increases
(C) stays the same increases
(D) increases stays the same
▶️Answer/Explanation
Ans: B
As the water streams out of the cart, the water is pushed out of the cart by pressure from above, exerting a reactionary force pushing the cart and its contents forward, increasing the speed of the cart. As the speed of the cart is increasing, and its mass remains the same, the kinetic energy of the cart must also increase.