Science
What is friction?
Quick answer
Friction is a force that acts when two surfaces rub or slide against each other. It slows things down and turns movement into heat, and it also stops us slipping.
In one sentence
Friction is a force that tries to stop two surfaces sliding past each other, slowing things down and making heat.
Explained
Push a book across a table and it soon stops. Something is acting against the movement. That something is friction, a force that appears whenever two surfaces rub together.
Even surfaces that look smooth have tiny bumps and dips. When they slide past each other, the bumps catch and grip. Imagine two hairbrushes pushed bristles-together and dragged apart.
Friction always acts in the opposite direction to the movement. It changes some of the energy of movement into heat, which is why rubbing your hands warms them up and why bike brakes get hot.
How much friction there is depends on the surfaces:
- Rough surfaces (sandpaper, carpet, tree bark) give lots of friction.
- Smooth surfaces (ice, polished floors) give less.
- Liquids like oil or water between surfaces can reduce friction, which is why wet floors are slippery.
Friction is not just a nuisance. Without it, you could not walk, because your feet would slide. You could not hold a pencil, and car tyres could not grip the road.
Air resistance and water resistance are similar forces. When something moves through air or water, the air or water pushes back. A parachute uses air resistance to slow a skydiver down while gravity pulls them towards the ground. Fish and boats have streamlined shapes to cut through water.
Examples
| Useful friction | Unhelpful friction |
|---|---|
| Shoe soles gripping the ground | Squeaky door hinges (fixed with oil) |
| Bike brakes stopping the wheel | Wearing out shoes and tyres |
| Matches lighting when struck | Machines heating up |
| Writing with a pencil | Pushing heavy boxes across the floor |
Common mistakes
- "Friction is always bad." We need friction to walk, grip, brake and write.
- "Smooth surfaces have no friction." All surfaces have some. Ice has very little, but not zero.
- "Friction only happens with solids." Air and water resistance act on things moving through gases and liquids.
Quick quiz
Friction
Check what you know with 4 quick questions.
Question 1 of 4
On which surface would a toy car roll the furthest?
Practise it
Try it
Drag a shoe across different surfaces using an elastic band and see how much it stretches. Then try more physics activities.
Friction and forces activities
Practise it step by step in a full course: Grade 4 Science
For grown-ups
Related topics
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A force is a push or a pull. Forces can make things start moving, speed up, slow down, change direction or change shape. They are measured in newtons.
Gravity
Gravity is the force that pulls objects towards each other. Earth's gravity pulls everything towards its centre, which is why things fall down and why the Moon orbits Earth.
Materials
Materials are what objects are made from, such as wood, metal, plastic and glass. Each has properties, like being hard, bendy, waterproof or see-through, that suit it to some jobs.
Simple machines
Simple machines are basic tools that make jobs easier by changing the size or direction of a force. The six types are the lever, wheel and axle, pulley, ramp, wedge and screw.
Stopping distance
Stopping distance is how far a vehicle travels from the moment the driver sees a hazard until it stops. Stopping distance = thinking distance + braking distance.
Electric circuits
An electric circuit is a complete loop that electricity flows around. A simple circuit has a battery, wires and a component like a bulb. If the loop is broken, the electricity stops.