Kiara and Rahim are on a train moving at 80 km per hour
Kiara and Rahim are on a train moving at 80 km per hour. Kiara says the passenger asleep in the opposite seat is not moving at all — he hasn't left his seat for an hour. Rahim says whether the passenger is moving depends on what you compare him with: compared with the trees outside, he is moving at 80 km per hour. Who is right?
Who do you think is right? Pick one.
The answer: Rahim is right, motion is always judged against a reference point
Why
Kiara is correct that the passenger's position hasn't changed compared with his seat, the carriage and the people around him — seen from inside the train, he is at rest. But that isn't the whole truth, because she is quietly using the train as her reference point without saying so. An object is in motion when its position changes with respect to a reference point. Compared with the trees, the platforms and the stations, the sleeping passenger's position is changing fast — in an hour he has travelled about 80 km without waking up. So 'is he moving?' has no single answer until you say 'compared with what?'. Test: look out of the window — the trees seem to rush backwards; to a person standing among those trees, it is you who rush past. Rahim is right: whether the passenger is moving depends on the reference point — at rest compared with the train, moving at 80 km per hour compared with the ground.
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