2 Definition of motion

Motion is the change in the position of an object with time, relative to the position of another object (a reference).

Two points to notice:

1. Motion is relative. A passenger sitting in a train is at rest relative to the train but moving relative to the platform. Whenever you say an object moves, you must say relative to what.
2. If the position of an object does not change over a period of time, it is at rest.

• When the object moves along a straight path, we call it motion in a straight line.
• It is the simplest type of motion.
• Example: a train on a long straight stretch of railway track.

(Think: if you look at a film strip showing the positions of a mouse at equal time intervals, the mouse is moving if its position relative to the background is different in the successive pictures.)