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?
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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.
The motion of everything in the Universe besides light can be described in this way.
The motion of everything in the Universe besides light can be described in this way.
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.
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.
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The correct answer is: If one object is moving much slower than the speed of light, its speed can be added to that of another object.
In this case, the train's motion and the people's motion are both much slower than the speed of light, so we can simply add their velocities together to describe their motion relative to the ground. Their total velocity is 10 m/s - 1 m/s = 9 m/s north, as given in the question. This is why we can use these motions to describe their motion relative to the ground.