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

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

If one object is moving much slower than the speed of light, its speed can be added to that of another object.
If one object is moving much slower than the speed of light, its speed can be added to that of another object.

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 motion of everything in the Universe besides light can be described in this way.

Answer 1

The correct response is:

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

This statement reflects the principle of relative motion, where you can add or subtract velocities based on a stationary frame of reference to determine the motion relative to the ground. In this case, the ground acts as the stationary frame of reference, allowing us to calculate the total velocity of the people relative to it by combining their speeds relative to the train and the speed of the train itself.