The Online Library of Knowledge

# What causes motion?

Motion is any kind of movement. Motion is caused by forces. A stationary object does not move unless a force acts on it to start it going. Once it is moving, it carries on at the same speed and in the same direction unless a force makes it speed up, change direction or slow down and stop.

## Different types of motion

This illustration shows examples of a number of different types of motion: linear motion (the football and slide), reciprocating motion (the swing) and rotary motion (the bicycle wheels and roundabout).

## Inertia

In the illustration above, a boy is being thrown off his skateboard. The board stopped suddenly when it hit a rock, but the boy himself continued to move forwards. This was because of inertia: the tendency for objects that are moving to carry on moving. The heavier the object, the greater its inertia.

If a bus stops suddenly, people on it fall forwards because there is nothing to stop them moving (although seat belts would hold them in place). This is another example of inertia.