What Is Motion?

Motion, a relative concept

An object is in motion when its position changes over time relative to a chosen fixed point. This fixed point is called the frame of reference. Without a frame of reference, it is impossible to say whether an object is moving or not.

Concrete example

A passenger sitting in a moving train is stationary relative to the train (train frame of reference), but is in motion relative to the trees along the track (ground frame of reference). Motion therefore always depends on the chosen frame of reference.

The trajectory

The trajectory is the set of successive positions occupied by an object during its motion. It can be:

Type of trajectory	Example
Rectilinear	A marble rolling along a ruler
Circular	A merry-go-round horse
Arbitrary	A tennis ball thrown into the air

When the trajectory is a straight line, the motion is said to be rectilinear.

Common pitfall to avoid

Do not confuse trajectory with distance traveled. The trajectory describes the shape of the path followed, whereas the distance traveled is a length measured in meters (m). Two objects can follow the same trajectory while covering different distances if one of them stops along the way.