Newton's Laws

These are an incredibly powerful tool to be used in tackling many types of problem and understanding what is going on in numerous situations. They are accepted as "laws", i.e. objects MUST obey these laws.

We live in a Newtonian universe where all objects obey these laws. It is often referred to as a "clockwork" universe. If we know the position and velocity of all the objects in a system now, e.g. our solar system, then we can calculate their position and velocity at any time in the future.

Law 1
An object will remain in a state of rest or unaccelerated motion unless acted upon by an external resultant force.

If an object isn't moving or if it is moving at a constant speed then all the forces acting on it must be balanced, i.e. there is no resultant force acting on it. The first question one might ask when tackling a problem is "is it in equilibrium?". If the answer is yes then, from Newton's 1st law we know that all the forces balance.

Law 2
If an external resultant force acts on an object it will produce a rate of change of momentum directly proportional to the force and in the same direction.

We don't meet momentum until year 13, but from this law comes the equation F = m a.

Law 3
If a body A exerts a force on body B then body B will exert an equal and opposite force on body A.

Forces come in equal and opposite pairs. If you push something it pushes back with an equal and opposite force. When an apple falls it is pulled towards the Earth by the force of gravity. An equal opposite force acts on the Earth pulling it upwards.

Consider these examples

These cars are travelling at a steady speed. There are several forces acting on them but we know that, either horizontally or vertically, all these forces balance. If they didn't then the cars would be speeding up or slowing down.	The acceleration of an object depends on its mass and the resultant force acting on it. These motorbikes have fantastic acceleration due to their small mass and powerful engines.	When you swim you move forwards because the water pushes you forwards. We make it do this by pushing the water backwards.