A railroad car of mass 12,000 kg is moving horizontally at 10 m/s. Suddenly it starts to rain, and water accumulates in the railroad car. Which of the following is true?
A) The railroad car slows down because the linear momentum of the rain + car system is conserved.
B) The railroad car continues to move at 10 m/s because the rain is falling vertically while the car is moving in a horizontal plane.
C) The railroad car continues to move at 10 m/s because the linear momentum of the rain + car system is conserved.
D) The railroad car continues to move at 10 m/s because the total energy of the rain + car system is conserved.
E) The railroad car speeds up because the potential energy of the rain is converted into kinetic energy for the car.

Question

Quizplus · Accepted Answer

The linear momentum of the rain + car system is conserved. When rain, which initially has no horizontal velocity, falls into the moving car, it adds mass to the car but does not add horizontal momentum. To conserve the total linear momentum of the system, the velocity of the car must decrease.

A Railroad Car of Mass 12,000 Kg Is Moving Horizontally