A freight car moves along a frictionless level railroad track at constant speed. The freight car is open on top. A large load of coal is suddenly dumped into the car. What happens to the speed of the freight car?
A) It increases.
B) It remains the same.
C) It decreases.
D) It becomes zero.
E) Cannot be determined from the information given.

Question

Quizplus · Accepted Answer

The speed of the freight car decreases because the mass of the system increases while the momentum is conserved. Since momentum (mass x velocity) is conserved and the mass increases, the velocity must decrease to keep the momentum constant.

A Freight Car Moves Along a Frictionless Level Railroad Track