What is a force?
A force is a push or a pull acting on an object. Forces cannot be seen, but we can see what they do. A force is measured in newtons (N) using a spring balance (force meter). Force has both size and direction, so it is drawn as an arrow.
Common forces around us
- Gravity is the pull of the Earth on every object, giving it weight. It always acts downwards, towards the centre of the Earth.
- Friction is a force that opposes motion between two surfaces that are touching. It acts in the opposite direction to movement.
- Normal force is the support force from a surface, and there are also pushing, pulling and stretching forces.
Key idea
Force is measured in newtons (N). Weight = mass × gravitational field strength. On Earth the field strength is about 10 N/kg, so a 2 kg bag weighs about 20 N.
Weight and mass
Mass is the amount of matter in an object, measured in kilograms, and it stays the same everywhere. Weight is the pull of gravity on that mass, measured in newtons, and it changes if gravity changes, for example on the Moon.
Example
Friction lets our shoes grip the floor when we walk, and lets car brakes slow the wheels. Without friction we would slip.
Remember
- A force is a push or a pull, measured in newtons (N).
- Gravity acts downwards and gives objects their weight.
- Friction opposes motion and produces heat.