CoachA reference frame is a rigid body — the ground, a train carriage, the Earth as a whole — relative to which positions are recorded, together with a clock to date them. A body is at rest in a frame when its position in that frame does not change over time; otherwise it is in motion in that frame.
Examples
Example 5.3 (The walker on the train)
A train travels at relative to the ground while a passenger walks toward the front at relative to the train.
- In the train frame: the passenger moves at down the aisle; the seats are at rest; the trees outside rush backward at .
- In the ground frame: the seats move at , the passenger at , and the trees are at rest.
The same scene, two frames, two entirely different descriptions — and no experiment performed inside the train can declare one of them “the real one”.
Example 5.9 (A hike with a break)
A hiker covers in of walking, then rests for , then covers in the final . Average speed over the whole outing:
The average counts all the elapsed time, breaks included — it is generally not the average of the speeds of the separate legs.
Example 5.14 (The moving walkway)
An airport walkway rolls at . A traveller walks on it at relative to the belt, in the direction of travel: speed relative to the ground . Walking the wrong way at the same pace: , still (slowly) progressing against the belt. Standing still on the belt: relative to the ground, relative to the belt — at rest and in motion at the same time, in two different frames.