A force can do one of four things to an object:

1. Make it speed up - accelerate.
2. Make it slow down - decelerate.
3. Change its direction.
4. Change its shape.

If something is doing one of these four things there must be a net force acting upon it.

(Note: Other names for a net force are an unbalanced force or a resultant force.)

What do we mean by a 'net' force?

Well, forces do not add up like normal numbers - you must take their direction into account as well. For example, if you were teetering on the edge of a cliff and someone applied a force to you, you would probably like the force applied in a certain direction.

It is easy to add up forces, just look at these three examples:

The unit we measure force in is the Newton, named after Sir Isaac Newton, which leads us onto Newton's first law.