1 What is force?

"Force" is a word we use every day: your muscles pull a bag, a car engine starts the car moving, a brake stops it.

Force is an external influence acting on an object that changes, or tries to change, its state of motion (its velocity in magnitude or direction).

• Force is a vector quantity (it has magnitude and direction).
• It is measured by a spring balance (spring scale).
• Its SI unit is the newton (N).

"Tries to change": if you push a heavy wall and it does not move, your push is still a force, but another force (the reaction of the floor/friction) balances it.