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What causes motion?

A space probe travels past JupiterA space probe travels past JupiterMotion 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

Different types of motion at a playgroundDifferent types of motion at a playground

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). 



The movement of a bus, demonstrating inertiaThe movement of a bus, demonstrating inertia

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.

An astronaut can place an object, for example, a tool, in space, and it just stays where it is. There is no force to cause it to move.

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