Resultant forces will cause acceleration, which can be described and calculated using Newton's laws of motion. Weight is caused by the gravitational effect of a planet attracting an object’s mass.

According to Newton's third law of motion, whenever two objects interact, they exert equal and opposite forces on each other.

This is often worded as 'every action has an equal and opposite reaction'. However, it is important to remember that the two forces:

- act on two different objects
- are of the same type (eg both contact forces)

Newton's third law can be applied to examples of equilibrium situations.

There are contact gravitational forces between Earth and the cat:

- the cat pulls the Earth up
- the Earth pulls the cat down

These forces are equal in size and opposite in direction.

There are contact forces between the person and the pram:

- the person pushes the pram forwards
- the pram pushes the person backwards

These forces are equal in size and opposite in direction.

There are contact forces between the tyre and the road:

- the tyre pushes the road backwards
- the road pushes the tyre forwards

These forces are equal in size and opposite in direction.

There are non-contact gravitational forces between Earth and the satellite:

- the Earth pulls the satellite
- the satellite pulls Earth

These forces are equal in size and opposite in direction.