What is friction?

What is friction?

Friction is a contact force: it only acts when two things touch. The AQA GCSE specification lists it alongside air resistance, tension and the normal contact force. Push a book across a table and let go, and friction from the table slows it down. Friction always acts along the surface, in the direction that opposes the sliding.

Like every force, friction is measured in newtons (N) and has a direction, so on a free body diagram it is drawn as an arrow along the surface, pointing against the way the object moves or is being pushed.

What causes friction?

Even surfaces that look smooth are rough under a microscope, covered in tiny bumps. When two surfaces touch, they only really meet at the tops of those bumps. Atoms at those contact points stick to each other, and the bumps catch on one another. Sliding means breaking those contacts over and over. OpenStax University Physics describes how the rubbing makes the atoms vibrate, which is why the energy ends up as heat. Rub your hands together to feel it.

Types of friction

What is static friction?

Static friction is the friction between two surfaces that aren't moving relative to each other. It is what stops something from starting to slide. Its most important feature is that it adjusts: push a heavy box gently and static friction pushes back just as hard, so nothing moves. Push harder, and it pushes back harder, up to a limit. Once your push beats that limit, the box starts to slide and kinetic friction takes over.

Kinetic friction is usually smaller than the maximum static friction. That is why a heavy box is hardest to get moving, and then suddenly easier to keep moving.

The friction formula

How big friction can get depends on two things: how hard the surfaces are pressed together, and what they are made of. In physics courses beyond GCSE, including AP Physics and A-level, this is written as:

• N is the normal force, how hard the surface pushes back on the object. On flat ground it equals the object's weight, mg.
• μ (the Greek letter mu) is the coefficient of friction, a number with no units that depends on the two materials. μ_s is for static friction and μ_k for kinetic.
• The ≤ sign means static friction can be anything from zero up to μ_sN, whatever is needed to stop sliding.

Some typical values, from OpenStax University Physics. They are approximate, because real surfaces vary.

Example 1: pushing a crate

A 20 kg wooden crate sits on a wooden floor, with μ_s = 0.5 and μ_k = 0.3. Take g = 9.8 N/kg. On flat ground, the normal force equals the weight:

The maximum static friction is 0.5 × 196 = 98 N. Push with 80 N and static friction pushes back with exactly 80 N, so the crate stays put. Push with more than 98 N and it starts to slide. Once it is sliding, kinetic friction is 0.3 × 196 = 58.8 N.

If you keep pushing with 98 N once it moves, the resultant force is 98 − 58.8 = 39.2 N. Dividing that by the mass gives the acceleration, which is Newton's second law in action: 39.2 ÷ 20 = 1.96 m/s². Check the sums in our Force Calculator.

Here x is the push in newtons and y is the friction force in newtons. The friction rises in step with the push while the crate is still, then drops as soon as it starts to slide. That drop is the jolt you feel when a heavy object suddenly gives way.

Try one: this question uses F = ma to link a resultant force, a mass and an acceleration.

Example 2: braking on a wet road

A car travelling at 20 m/s (72 km/h, about 45 mph) skids to a stop on a level road. If friction is the only horizontal force, then F = μmg and F = ma, so the mass cancels and the deceleration is a = μg. Using a mid value of μ_k = 0.4 for a wet road and 0.7 for a dry one:

Less friction on the wet road means the car slides about 22 m further. That is why AQA lists bad road and weather conditions, and worn tyres or brakes, as things that increase braking distance. The distance comes from the SUVAT equation v² = u² + 2as, covered in our SUVAT equations guide.

Air resistance and terminal velocity

Air resistance, or drag, is friction from the air, and unlike sliding friction it grows as you go faster. A skydiver who jumps from a plane speeds up at first because their weight is much bigger than the air resistance. The faster they fall, the bigger the drag, until it balances their weight. The forces are then balanced, so they stop accelerating and fall at a steady top speed called terminal velocity.

Opening the parachute gives the air a far larger area to push on, so drag suddenly outweighs the skydiver, who slows down until the forces balance again at a new, lower terminal velocity that is safe to land at. Try it: this question asks you to read what is happening at a point in a skydive.

Is friction good or bad?

Both. Life without friction would be impossible, but in machines it wastes energy.

How to reduce friction

• Lubricate. A layer of oil keeps surfaces apart. In the table, oiling steel on steel cuts μ_s from 0.6 to 0.05. Your joints use synovial fluid the same way.
• Roll instead of slide. Wheels and ball bearings replace sliding friction with much smaller rolling friction.
• Make surfaces smoother by polishing them.
• Streamline to cut air or water resistance, which is why racing cars and fish have smooth, pointed shapes.

To increase friction, do the opposite: grippy rubber soles, treads on tyres, grit on icy roads. The difference matters: in the table, shoes on wood have a static coefficient 9 times bigger than shoes on ice.

Common mistakes

• Thinking friction always opposes motion. It opposes sliding. When you walk, friction on your shoe points forwards, the way you are going, because it stops your foot slipping backwards.
• Using μ_sN as the friction on a still object. That is only the maximum. If you push with 30 N and it doesn't move, the friction is 30 N.
• Saying a moving object needs a force to keep moving. It needs a force only to balance friction. With no friction, it would keep going.
• Thinking friction depends on contact area. For most solid surfaces it depends on the normal force and the materials, and hardly at all on the apparent area.
• Giving μ a unit. It is a ratio of two forces, so it has none.
• Forgetting that friction heats things. The energy isn't lost; it ends up warming the surfaces, in their thermal store.

Read next

Physics · MechanicsNon-contact forces and contact forces: GCSE examplesNon-contact forces act at a distance; contact forces need touching. See examples of each, whether upthrust is a contact force, and GCSE worked examples.