An open freight car rolls friction free along a horizontal track in a pouring rain that falls vertically. As water accumulates in the car, the car's speed
A)increases.
B)decreases.
C)remains the same.
This is best explained by the principle of
D)inertia.
E)momentum conservation.
F)energy conservation.
G)Newton's 2nd law.

Question

Quizplus · Accepted Answer

The car's speed decreases because as rainwater accumulates, the total mass of the car plus water system increases. According to the principle of momentum conservation, if no external horizontal forces act on the system, the total momentum of the system must remain constant. Since momentum is the product of mass and velocity, an increase in mass (due to accumulating water) must result in a decrease in velocity to keep the momentum constant.

An Open Freight Car Rolls Friction Free Along a Horizontal