Rest and Motion: Motion is Relative

Key points

• An object is at rest if its position does not change with time, relative to a reference point.
• It is in motion if its position changes with time, relative to that point.
• The same object can be at rest for one observer and moving for another.
• Nothing is in absolute rest: even the ground moves with the Earth around the Sun.

Relative to whom?

Sitting in a moving train, are you at rest or moving? Both answers are right, because rest and motion are relative: they mean nothing until you choose a reference point (an observer or origin) to measure position from.

Key definitions

Position changes with time → motion. Position does not change → rest.

More examples

• Passengers in a bus are at rest relative to each other but moving relative to the road.
• A tree is at rest relative to the ground but moving relative to the Sun.
• A geostationary satellite stays over one spot of Earth, yet moves fast relative to the Sun.

Common mistake: saying something is simply "at rest" or "moving". Always name the reference point. Slow motion is still motion if the position changes.

Try it yourself

Goal: See that the same passenger is at rest for one observer and in motion for another.

• Choose who is watching: the person on the platform or your friend in the train.
• Press Play (or drag the time slider).
• Watch your position measured from that observer: does it change?
• Switch observers and compare.

Open the simulation full screen

Quick quiz

4 questions. Choose an answer for each, then submit to see your score and the explanations.