The big idea: Try to shove a heavy sofa. At first it won't budge — then suddenly it gives and slides more easily.
That's friction: the force between two touching surfaces that opposes sliding. It comes in two kinds — static (holding a still object in place) and dynamic (once it's sliding).
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Static friction (not moving)
- Acts while the object is still stuck
- Grows to match whatever pushes it
- Has a maximum of μs R — beyond that it slips
- Rule: Ff ≤ μs R
Dynamic friction (sliding)
- Acts while the object is sliding
- Has a fixed size
- Usually smaller than the max static friction
- Rule: Ff = μd R
Forces on a box on a rough surface: your push (right) and friction (left) act along the surface — friction always opposes the push. Weight (down) and the normal force R (up) act perpendicular to it.
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Spot the difference: Static = stuck (friction can be anything up to μs R). Dynamic = sliding (friction is exactly μd R).
The normal force R (also written FN) is the surface pushing back, at a right angle to the surface.
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Both friction rules are given in the data booklet. The normal force R is how hard the surface pushes back; on flat ground R is just the object's weight, R = mg.
- friction force, parallel to the surface (N)
- coefficient of STATIC friction (no unit — just a number)
- normal force — the support push, perpendicular to the surface (N). Also written F_N.
- friction force while sliding, parallel to the surface (N)
- coefficient of DYNAMIC (kinetic) friction (no unit — just a number)
- normal force — the support push, perpendicular to the surface (N). Also written F_N.
Why is μ just a number?: Rearrange Ff = μR to μ = Ff ÷ R — that's a force ÷ a force. The newtons on top and bottom cancel, so μ has no unit: it is dimensionless (a pure number, usually between 0 and 1).
Friction triangle: Ff = friction, μ = coefficient, R = normal force. Cover the one you want — two side by side → multiply (Ff = μR); one above the other → divide (μ = Ff ÷ R).
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A crate of mass 20 kg is dragged across a flat floor at steady speed. The coefficient of dynamic friction is μd = 0.30. (Take g = 9.8 m s⁻².) Find the friction force. (On flat ground R = mg.)
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How this is tested — friction shows up in force and motion questions. It comes up two ways:
Paper 1A
- Quick concept — why μ is dimensionless.
- Static vs dynamic friction.
Paper 1B / Paper 2
- The classic 'show that' calculation.
- The minimum force to start an object moving.
The classic trap: To start motion you must beat the largest static friction (μs R), not the smaller sliding friction. Use μs, not μd.
Minimum force to start moving: An object stays still until your push beats the biggest static friction it can muster. So the minimum force to start it sliding equals μs R — the maximum value of Ff ≤ μs R.
A box of mass 1.2 kg sits on a flat bench. The coefficient of static friction is μs = 0.35. (Take g = 9.8 m s⁻².) Show that the minimum horizontal force to start it moving is about 4 N.
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