In any given situation the forces will combine (add together) and/or cancel. The result may be an unbalanced force which will cause acceleration. If the forces combine and cancel each other

then the overall force will be zero.

Example

The engine thrust is greater than the friction from the water which means there is an overall unbalanced force directed forwards. This means the speed boat will accelerate.To determine the size of the

unbalanced force:

unbalanced force = engine thrust force – frictional force

Newton’s Laws of Motion

1. An object will remain stationery or continue at constant speed if no external forces act.

2. An object will accelerate if an external force acts on the object.

Force = Mass *x* Acceleration

Newtons (N)

Mass kilograms (kg)

Acceleration (rns^{-2})

**Worked Example**

If a block of 0.5 kg is pushed and accelerates sideways along a bench at 2ms^{-1}. What is the size of the force that was applied to the block?

F = m. a

F = 0.5 *x* 2

= 1.0 N

Newtons 3rd Law

For every action there is an equal and opposite reaction. This means if a force is pushing an object the object will push back with an equivalent force.