Skip to content

Static and kinetic friction

Forces and motion · level 6, lesson 1 of 6

A question to start

Push a heavy wardrobe. At first it will not budge. You push harder and harder; suddenly it gives, and now it slides on with a smaller push than you needed to start it. Why is starting harder than keeping going?

The idea

There are two kinds of friction.

  • Static friction acts between surfaces that are not sliding. It matches whatever force is trying to slide them, up to a maximum. Push gently, and static friction pushes back just as gently; the wardrobe stays still. Push harder, and it pushes back harder, until you reach its limit.
  • Kinetic friction (or sliding friction) acts once the surfaces are sliding. It stays about the same at any speed, and is usually smaller than the maximum static friction.

That is why the wardrobe needs a big push to start and a smaller one to keep going.

push appliedfriction static: matches the pushmaximumkinetic: slidingnot slidingsliding

Why the difference? When two surfaces sit still, their tiny bumps settle into each other and may stick slightly. Once sliding, the bumps skip across each other with less time to catch.

Static friction is the useful one in walking and driving. A rolling wheel's contact point is not sliding, so the grip between a tyre (or a rover wheel) and the ground is static friction. When a wheel spins and slides, grip falls to the smaller kinetic value. That is why anti-lock brakes on a car stop the wheels from locking and sliding.

Worked examples

1. A crate. Its maximum static friction is 300 N and kinetic friction is 220 N. Push with 200 N: it stays still, friction is 200 N. Push with 310 N: it starts sliding, and now the net force is \(310 - 220 = 90\) N.

2. A wheel in sand. A rover wheel gripping firm ground can push with up to the static limit. If it starts to spin in loose sand, the grip drops, and it digs in. Drivers on Mars slow the wheels to keep them below slipping.

Try it

  1. A box has a maximum static friction of 50 N. You push with 30 N. What is the friction force?
  2. You push with 60 N, and the kinetic friction is 40 N. What is the net force once it slides?
  3. Is the grip between a rolling tyre and the road static or kinetic friction?
  4. Why is a sliding (locked) wheel worse at braking than a rolling one?
Answers
  1. 30 N: static friction matches the push.
  2. \(60 - 40 = 20\) N.
  3. Static: the contact point is not sliding.
  4. Kinetic friction is smaller than maximum static friction, so a sliding wheel grips less.

Common mistake

"Static friction is always at its maximum." Static friction is only as big as it needs to be to stop sliding. On a box nobody pushes, on flat ground, it is zero.

Where it leads