Friction: the force that tends to stop things

Slide a book across a table

Push a book across a table: it slides, then stops. The table stopped it, with a force called friction.

Friction acts between two surfaces that touch, against sliding. No surface is perfectly smooth: tiny bumps catch on each other.

Friction when nothing moves

Push a heavy crate gently with $100\ \text{N}$ and it stays put: the floor's friction pushes back with $100\ \text{N}$. Push with $150\ \text{N}$: friction grows to $150\ \text{N}$. Friction matches your push, up to a limit. Push beyond that limit and the crate starts to slide.

Friction when it slides

Once the crate slides, friction is usually a little less than the limit. That is why it is hardest to get something started.

You should find that the friction line rises with your push, then drops to a lower, steady value once the crate slides. Only then is there a net force and an acceleration.

Useful and harmful

Friction lets you walk, lets tyres grip and brakes work. It also wears out shoes and machines and wastes energy as heat. We increase it with treads and grit, and reduce it with oil, smooth surfaces, ball bearings and wheels.

Watch out. Friction does not always point backwards. It points against the sliding: when you walk, it pushes your foot forward.

A worked example

A $40\ \text{kg}$ crate: friction can hold up to $200\ \text{N}$, and is $160\ \text{N}$ once it slides. What happens for pushes of $100\ \text{N}$, $180\ \text{N}$ and $240\ \text{N}$?

1. $100\ \text{N}$ and $180\ \text{N}$. Below the limit: friction matches the push and the crate stays still.
2. $240\ \text{N}$. Above the limit, so it slides: net force $240\ \text{N} - 160\ \text{N} = 80\ \text{N}$.
3. Acceleration. $\dfrac{80\ \text{N}}{40\ \text{kg}} = 2\ \text{m/s}^2$.

Practise with new numbers

Try it in Python

The program works out the three pushes.

Remember these

• Friction acts between surfaces that touch, against sliding.
• It matches a push up to a limit; sliding friction is usually a little less.
• Increase it with grip; reduce it with oil, wheels and ball bearings.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why are the soles of sports shoes ridged?

2. A box stays still when you push it with $70\ \text{N}$. How big is the friction on it?

3. Why is it harder to start pushing a heavy cupboard than to keep it moving?

4. Give two ways to reduce friction in a machine.