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Rest and motion
Beginner
An object moves if its position changes relative to a reference frame.
Rest and motion are relative: an object is moving only compared with some reference. A passenger is at rest relative to the train but moving relative to the platform. Describing motion therefore always needs a frame and a clock.
A book on a table is at rest relative to the room but moves at about 465 m/s relative to Earth's axis at the equator.
Saying "it is moving" without naming a reference is incomplete.
In more detail
Whether an object is moving depends on the reference point you choose. A passenger is at rest relative to the train but moving relative to the ground.
Real-world uses
- Satellite operators describe orbits relative to Earth's centre, not the ground under a station.
- Passengers on a moving bus judge motion relative to the seats, not the road.
- A sailor on a ship sees the deck as still, while a harbour official on the shore sees the same ship moving at full speed.
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Frequently asked questions
Is anything truly at rest?
No. Motion is always relative to something, so 'at rest' means at rest relative to a chosen frame.
Does the choice of reference matter?
Yes. The same motion can look simple in one frame and complicated in another.
If I am sitting still, am I moving?
You are at rest relative to your chair and the room, but moving relative to the Sun. Both statements are true in their own frames.
Related topics in Kinematics
Reference frames · Distance vs displacement · Speed vs velocity