What can forces do? A force can do one of four things to an object: Make it speed up - accelerate. Make it slow down - decelerate. Change its direction. 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.) Net forces 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.