Unit 2, lesson 9. About 15 minutes.

Friction

Why would you slip over if the floor had no friction?

Play

Getting the experiment ready…

Explain

Rough things grip.

Slippery things slide.

Friction is a force between two surfaces that rub together. It always works against sliding.

If you look very closely, even smooth surfaces have tiny bumps that catch on each other.

Friction can be useful. Your shoes grip the floor so that you can walk.

The friction force grows to match your pull, up to a limit. Past that limit, the object slides.

The limit is F = μ × m × g on level ground. Here μ, the Greek letter mu, is the coefficient of friction.

Worked example: a 10 kg box has μ = 0.3. So the friction limit is 0.3 × 10 × 9.8 = 29.4 N.

Surfaces touch
Tiny bumps catch
Friction resists sliding

Want the full story?

Real surfaces need more force to start sliding than to keep sliding. These are called static and kinetic friction, but the sim uses a single value for both. Friction depends on the force pressing the surfaces together, called the normal force, which equals weight only on level ground.

The Formula

friction = grip of the surface × mass × gravity

F_f = \mu \times m \times g

$F_f$: the friction force, measured in newtons (N)
$\mu$: the coefficient of friction, a number for how grippy the surfaces are, which has no unit
$m$: the mass of the object, measured in kilograms (kg)
$g$: the strength of gravity, 9.8 on Earth, measured in newtons per kilogram (N/kg)

Try the numbers

Grip: 0.3Mass: 10 kg

Friction force: 29.43 N

Try it at home

You need a shoe, a smooth table, some paper and some cloth.

You need: a shoe, a smooth table, some paper and some cloth.

Slide a toy on a cloth. Now on a table.

Push a shoe gently along a smooth table.
Put a cloth down and try it again.
Put a book in the shoe and try again.

What's happening: rougher surfaces and heavier objects both make more friction.

Physics is life

Your body

Your skin has grip lines on your fingers.

The ridges on your fingertips are thought to help you grip rough or damp things.

Daily life

Brakes use friction to stop.

Bike and car brakes use friction to slow the wheels, and tyres grip the road.

Nature

Snakes grip with their scales.

A snake uses friction between its belly scales and the ground to push itself along.

Space

Mars rovers need grippy wheels.

Mars rovers have ridged wheels, which grip the loose ground as they climb.

Quick check

Question 1 of 3

Which way does friction act?

Words to know

Friction: A force between surfaces that rub together. It resists sliding.

Friction: A force between surfaces that rub together. It resists sliding.
Grip: How well two surfaces hold on to each other.
Force: A push or a pull on something.

Next up

Air resistance and terminal velocity