What is friction?

Friction is the force between two surfaces that resists them sliding over each other. It is a force, so it is measured in newtons (N), and you can measure it with a spring scale (see ForceClear).

Hook a block to a spring scale and pull gently. Nothing moves, because the table pulls back on the block with exactly the same force. That is static friction: it grows to match your pull, but only up to a limit. The limit is μs × N, where N is the normal force pressing the surfaces together (on a flat table, the block's weight) and μs, "mu-s", is the coefficient of static friction. μ has no unit: it is a ratio, friction ÷ normal force.

Pull past the limit and the block breaks free. Now kinetic friction (sliding friction) takes over, and it is usually smaller: μk × N. That is why the scale reading drops the moment the block starts to slide, and why a heavy cupboard is hardest to shift at the very start.

Add weight and N grows, so friction grows in step: double the weight, double the friction. The size of μ depends on the pair of surfaces: about 1 for rubber on tar, 0.45 for dry wood, 0.04 for Teflon and 0.03 for steel on ice.

Two neat tricks. Tilt the table until the block just starts to slide: at that angle tan θ = μs, and the block's weight doesn't matter. And zoom in: even polished steel is a mountain range. The surfaces touch only on the tips of the bumps, called asperities, and the real contact is often less than a millionth of the area you see.

A car's engine turns the wheels, but what actually pushes the car forward is the road, through friction on four patches of rubber, each about the size of your hand. The same grip lets you turn and stop. With no friction, like on sheet ice, the wheels just spin (see CarClear and MotorcycleClear).

A rolling tyre doesn't slide on the road: the bit touching the ground is momentarily still. So the grip is static friction, and it is bigger than sliding friction. Typical tyre grip, μ: about 0.8 on a dry road, 0.5 in the wet, 0.1 on ice. The shortest stop is a steady μ × g of deceleration, and the braking distance is d = v² ÷ (2 μ g). Double the speed and you need four times the distance. On ice it is eight times longer than on a dry road.

Stamp too hard and the wheels lock and skid. Now it is kinetic friction: grip drops by about a fifth, and a sliding tyre can't steer. ABS (anti-lock brakes) watches each wheel and releases the brake up to about 15 times a second, holding the tyre just short of skidding, at the top of its grip.

In the rain, the tyre must push water out of the way: at 80 km/h a new tyre clears up to 30 litres a second through its tread grooves. Worn tread can't, water builds up in front, and the tyre rides up on a film of water: aquaplaning. There's nothing to grip, so you can't steer or brake.

A bicycle's rim brakes squeeze rubber pads onto the wheel rim (see CycleClear). Friction at the rim turns into friction at the road. Wet rims grip far less until the pads wipe them dry.

Friction turns useful work into heat. Engineers spend a lot of effort fighting it, and they have three big tricks.

Roll instead of slide. A wheel doesn't rub the road; it just squashes a little as it turns, and some energy is lost in the squashing. That is rolling resistance, F = Crr × m g, and Crr is tiny: about 0.01 for car tyres, 0.004 for a racing bike, 0.001 for a train wheel on a steel rail. One person can push a 1,200 kg car on a flat road (about 120 N), but could never drag it with the wheels locked (over 8,000 N).

Put balls in the bearings. A spinning shaft needs a bearing to hold it. A plain bush rubs; a ball bearing lets hardened steel balls roll between two rings, with a friction coefficient of about 0.0015. That is why a ceiling fan spins for so long after you switch it off, and why a mixer grinder's motor can turn 18,000 times a minute without cooking itself (see FanClear and MixerClear).

Float it on oil. Where parts must slide, like the crankshaft in a car engine (see CarClear), engine oil is dragged into the gap and builds up a pressure that lifts the shaft off the metal on a film a few thousandths of a millimetre thick. The Stribeck curve shows how friction plunges once that film forms. At start-up the film isn't there yet, which is when engines wear most.

Whatever friction is left becomes heat. Rub your hands and you make about 2.5 W. Stop a car from 100 km/h and its front brake discs warm by about 40 °C; stop again and again with no time to cool and they glow red (see HeatClear).

We fight friction in machines, but we depend on it everywhere else.

Walking. To step forward, your shoe pushes the ground backwards, and friction from the ground pushes you forwards (Newton's third law, see NewtonClear). A normal step needs a friction coefficient of about 0.18. Dry concrete gives 0.7, so no problem. Wet polished tiles give about 0.2 and ice about 0.1: hurry, and your foot shoots out. That is why people shuffle like penguins on ice: short steps need less grip.

Writing. The tiny ball in a ballpoint pen (see PenClear) only turns because the paper grips it. As it rolls, it picks up ink from inside and lays it on the page, turning about 4.5 times for every centimetre. On glass it just skids and nothing comes out.

Knots, nails and screws. A knot holds because the rope presses on itself, and friction multiplies. Wrap a rope round a post and the friction grows exponentially with each turn: T = T₀ × e^(μθ), found by Euler. Three turns let one hand hold a boat. A nail holds only by friction from the squeezed wood fibres; a screw adds its threads.

Matches. Striking a safety match rubs the head on the box. Friction heats a speck of the box's red phosphorus enough to turn it into a far more touchy form, which catches fire and lights the head.

Music. A violin bow is coated in sticky rosin, so its static friction is far bigger than its sliding friction. The bow grabs the string, drags it sideways, the string slips back, and the bow grabs it again: stick–slip, hundreds of times a second. The same jerky dance makes chalk squeak, doors creak, brakes squeal and faults in the Earth snap in earthquakes.

Myth: more contact area means more friction. It feels obvious, but slide a brick flat or on its edge and the pull is the same. That is Amontons's law (1699): friction depends on the load, not on the apparent area. The reason is the tiny real contact you saw in chapter 1. Spread the brick over a bigger area and each bump is pressed less, so fewer bumps touch: the real contact area stays about the same. Rubber tyres break the rule a little, because rubber is soft and sticky.

Myth: smoother always means less friction. Up to a point. Polish two clean metals too far and they touch over so much area that the atoms stick. In a vacuum, clean metals can even cold-weld. Polished steel gauge blocks "wring" together so hard you must slide them apart. In 1991 the Galileo spacecraft's big antenna jammed half-open on the way to Jupiter: dry lubricant had worn off a few metal pins, and bare metal gripped bare metal.

Myth: ice is slippery because your weight melts it. Pressure does lower ice's melting point, but only by about 0.0074 °C per atmosphere. A skater's blade lowers it by about 1.5 °C, yet skating works at −20 °C. The real reasons: ice has a naturally wet, loose surface layer a few molecules thick (Faraday guessed it in 1850), and sliding heats the surface. Ice is slipperiest near −7 °C, which is why skating rinks are kept close to that.

Superlubricity. Twist one sheet of graphite a few degrees on another and their atomic bumps can't lock together: friction almost vanishes. Line them up again and it comes back.

Air resistance is friction with a fluid (see BernoulliClear). Unlike dry friction, it grows with speed squared. Above about 15 km/h, a cyclist fights the air far more than the road.