Two people are on a train that is moving at 10 m/s north. They are walking 1 m/s south relative to the train. Relative to the ground, their motion is 9 m/s north.

Question

Why are we able to use these motions to describe the motion relative to the ground?

(1 point)
Responses

As long as the frame of reference is stationary, all motion can be described in this way.
As long as the frame of reference is stationary, all motion can be described in this way.

The people are moving much slower than the speed of light so the ground acts as a frame

Answer 1

The correct response is:

As long as the frame of reference is stationary, all motion can be described in this way.

In this scenario, the train serves as a moving frame of reference, and the ground is a stationary frame of reference. The velocities can be added or subtracted to determine the overall motion relative to the ground, since classical mechanics allows us to analyze motion this way as long as we are not approaching relativistic speeds (which is not the case here, as the speeds involved are significantly less than the speed of light).