Static Friction
A self-adjusting force that holds things together, up to a limit.
Lesson at a glanceShow the key points
- Static friction acts between surfaces that do not slip; it adjusts its size and direction to prevent slipping.
- It cannot exceed the limiting friction: fₛ ≤ μₛN, where μₛ is the coefficient of static friction.
- Usually μₛ is slightly larger than μₖ, which is why it is harder to start a push than to keep sliding.
- Friction opposes RELATIVE motion, not motion; static friction from the road is what accelerates a car.
- To find its direction, imagine the surfaces frictionless: friction opposes the slipping that would result.
Three friends lean on a loaded steel cupboard and it doesn’t budge a millimetre. Something is pushing back exactly as hard as they push. That something is static friction.
A force that adjusts itself
Surfaces in contact that are not slipping can still exert friction on each other. The cupboard is at rest, so the net force on it is zero: the floor must be pushing it back, along the floor, as hard as the friends push it forward. This is static friction.
Push gently and static friction is small; push harder and it grows to match. It is self-adjusting: it takes whatever size and direction are needed to keep the two surfaces at rest relative to each other. But only up to a point.
Once the applied force exceeds the limiting friction , static friction can no longer keep up, and slipping begins. From then on kinetic friction takes over. Usually is a little larger than , which is why getting something moving takes more effort than keeping it moving. Like kinetic friction, the limiting friction does not depend on the contact area for a given normal force.
A 25 kg child sits on the rear seat of a scooter that speeds up at 1.5 m/s². (a) If the child doesn’t slide, what friction does the seat exert? (b) If the child does slide back, what can you say about μₛ? Take g = 10 m/s².
Which way does static friction point?
Use Newton’s laws. Put block A on block B on a table and pull B to the right. If nothing moves, A is in equilibrium and no other horizontal force acts on it, so the friction on A is zero. Pull harder so that both blocks accelerate together to the right: A now needs a net force to the right, and friction is the only candidate. So friction on A points right, in the direction of motion.
A second trick for the direction: imagine the surfaces were frictionless for a moment. Work out which way they would slip relative to each other; static friction points against that would-be slipping.
- 1Static friction acts without slipping and adjusts its size and direction as needed.
- 2: the limiting friction is a maximum, not the usual value.
- 3Beyond the limit, slipping starts and kinetic friction μₖN (usually a bit smaller) takes over.
- 4Friction opposes relative slipping, and often drives motion (walking, cars, conveyor belts).
Check yourself
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