Chapter 7, Problem 1OQ

A railroad car loaded with rocks coasts on a level track without friction. A worker at the back of the car starts throwing the rocks horizontally backward from the car. Then what happens?

1. The car slows down.
2. The car speeds up.
3. First the car speeds up and then it slows down.
4. The car's speed remains constant.
5. None of these.

To determine

What happens to the speed of the railroad car, when a worker starts throwing rocks horizontally backward from the railroad car loaded with rocks.

Answer to Problem 1OQ

Solution:

The correct option is (B)The car speeds up.:

Explanation of Solution

Given:

The truck is loaded with rocks and is moving on a level track without friction.

The worker throws rocks horizontally in the backward direction.

The rock and the railroad car move forward with the same speed. Hence they have the same velocity with respect to ground and are at rest with respect to each other. In the system comprising of the railroad car and the stone, the total initial momentum of the car and the stone is zero.

When the stone is thrown backward, it acquires a velocity in the opposite direction to that of the car. The stone, thus has a momentum in the opposite direction to that of the car. Since, in an isolated system, momentum is conserved, the railroad car acquires an equal momentum in the forward direction. The speed of the car increases in the forward direction due to increase in its momentum. The car therefore speeds up.