Friction: Friend or Foe?

Find out how friction helps us grip, walk and stop, and how we reduce it when it wastes energy.

About 3 min read
  • Describe what friction does
  • Give examples of useful and unwanted friction
  • Explain how to increase or reduce friction
Before you start

You know that a force is a push or a pull, and that forces can start, stop or change the movement of objects.

What is friction?

Friction is a force between two surfaces that slide or try to slide past each other. Even surfaces that look smooth have tiny bumps. These bumps catch on each other and resist movement. Friction always acts in the opposite direction to the movement, so it slows moving objects down.

Definition: friction

A force that resists movement between two surfaces in contact.

When friction helps

Friction lets your shoes grip the floor when you walk. Without it, your feet would slide as they do on smooth ice. Bicycle brakes press pads against the wheel, and friction slows the bicycle. Car tyres grip the road so that the car can turn and stop safely.

When friction causes problems

Friction can wear surfaces away and make moving parts hot. It also means engines need more energy. We reduce friction with a lubricant such as oil, smooth surfaces, or wheels and ball bearings.

Definition: lubricant

A substance such as oil that reduces friction between surfaces.

Explain a choice of shoe

Why do football boots have studs?

Grass can be slippery, so ordinary flat soles may slide.

Studs press into the ground and increase the grip.

Answer: Studs increase friction between the boot and the ground, so the player is less likely to slip.

Rubbing creates heat

Rub your hands together quickly and they feel warm. Friction transfers some energy to heat.

Exam focus
Say it like this“Oil reduces friction between the moving parts.”
Not like this“Oil stops friction.” — A lubricant reduces friction but does not remove it completely.
Common mistakeSaying that friction is always bad. It is useful when we need grip or need to slow down.

Practice

In which direction does friction act on a sliding box?
Opposite to the direction in which the box is moving.
Give one way friction is useful on a bicycle.
Brake pads use friction to slow the wheel. Tyres also use it to grip the road.
Why do people oil a squeaky door hinge?
Oil reduces friction between the moving metal parts, so they move more easily and quietly.
Going further
A car drives onto an icy road. Explain why it takes longer to stop.
Ice is smooth, so there is less friction between the tyres and the road. The force slowing the car is smaller, so stopping takes longer.

Quick check

Friction quick check

Which surface gives the most friction for a sliding shoe?

Its bumps catch against the shoe sole more strongly than the smooth surfaces do.

What can friction do to moving parts?

Surfaces rubbing together slowly lose material and transfer energy to their surroundings.

Which would reduce friction in a machine?

Oil separates the moving surfaces, so their tiny bumps catch less.
Key takeaways
  • Friction resists movement between surfaces.
  • It helps us grip, walk, turn and stop.
  • It can also cause wear and heating.
  • Lubricants and smooth surfaces reduce friction.
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