Newton developed __three laws of motion:__

- An object will remain at rest or continue to move at a constant velocity unless acted upon by a resultant force.
- The net (resultant) force acting on an object is directly proportional to the rate of change of its momentum, and is in the same direction.

- When two objects interact, they exert equal and opposite forces on each other. For every action, there is an equal and opposite reaction.

__Linear momentum:__

This depends on the object’s mass and velocity.

The SI unit of momentum is kgms^{-1}. It is a vector quantity since it is the product of a scalar (mass) and vector (velocity). Remember to include a negative sign if applicable.

__Force and momentum:__

Force and momentum are linked by Newton’s second law (see above): namely, the net force is equal to the rate of change of momentum.

__Impulse of a force:__

The impulse of a force is defined as the product of force and the time over which the force acts on an object. It is the change in momentum, and its unit is Ns or kgms^{-1}.

The impulse (and hence change in momentum) is equal to the area under a force-time graph.