Motion and Rest

Imagine yourself sitting in a seat while travelling in a moving train. You observe no change in position with respect to the window. There is change of scene when you view through the window. The change of scene indicates that the train is moving.

An object is said to be in motion if it changes its position with respect to its surroundings in a given time.

We know that the window in the cabin is at rest i.e., its position with respect to the walls of the cabin does not change with time.

An object is said to be at rest if it does not change its position with respect to its surroundings.

Have you watched the night sky? You have observed that the position of stars and planets change while you remain stationary. In reality the earth is moving too. Thus, an object which appears to be at rest, may actually be in motion. Therefore, motion and rest are relative terms. To describe the motion of an object we have to specify how its position changes with respect to a stationary object. This is called the frame of reference.

A frame of reference is another object or scene with respect to which we compare an object's position.

Example for Frame of Reference:

Figure 1                                                                   Figure 2

Look at the figures. In figure1, the car is to the right of the tree. In figure 2, after 2 seconds, the car is to the left of the tree. As the tree does not move, the car must have moved from one place to another. Therefore, here the tree is considered as the frame of reference.