Question

force and motion concepts quick check
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.
why are we able to use these motions to describe the motion relative to the ground?
(1 point)
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.
the motion of everything in the universe besides light can be described in this way.
the people are moving much slower than the speed of light so the ground acts as a frame of reference.

Explanation:

In classical mechanics (valid for speeds much lower than the speed of light), velocities can be added when considering relative motion with respect to different frames of reference. Here, the ground serves as a frame of reference and since the speeds involved are much less than the speed of light, we can add/subtract the velocities of the train and the people on it to find the velocity relative to the ground.

Answer:

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