Friction

Friction is the force that resists motion when two surfaces rub against each other. It is what slows a rolling ball, lets you walk without slipping, and stops a sliding box. Friction can be helpful (you could not write with a pencil without it) or annoying (your bike wheel does not roll forever because of friction). Engineers spend huge amounts of time both reducing friction (in engines and bearings) and increasing it (in brakes and tyres). Friction is one of the most useful and most universal forces in everyday life.

  • What it doesResists motion between surfacesAlways opposes motion
  • DirectionOpposite to motionAlways pushing back
  • TypesStatic, kinetic, rolling, fluidEach behaves differently
  • CauseRough surfaces + stickingEven smooth surfaces have it
  • Reducing frictionOil, wheels, ball bearingsLubrication and rolling
  • Increasing frictionTread, grit, brake padsWhere grip matters

What causes friction?

If you look at any surface (even something that feels perfectly smooth) under a microscope, it is bumpy. Tiny ridges, pits and rough patches cover every surface. When two surfaces are pressed together, the tiny bumps catch on each other. To slide one past the other, you have to push hard enough to break or jump over the bumps. That is friction.

There is also a separate kind of friction caused by molecules sticking together (sticking is what makes superglue work; on smaller scales it makes anything stick weakly).

Types of friction

  • Static friction: the friction between two surfaces that are not yet moving against each other. Usually the biggest type. It is what stops a parked car from rolling on a hill.
  • Kinetic friction (sliding friction): the friction between two surfaces sliding past each other. Almost always smaller than static friction. That is why something starts to slide more easily once you have pushed it past the first jolt.
  • Rolling friction: the friction when a wheel or ball rolls along a surface. Much smaller than sliding friction. This is why wheels were such an important invention.
  • Fluid friction: the friction of moving through a liquid or gas. Air resistance and water resistance are examples.
Fact Friction can heat things up. When you rub your hands together quickly, your skin warms. When a car brakes hard, the brake discs glow red-hot. When a meteor enters the atmosphere, friction with the air burns it into a "shooting star". All the energy used to overcome friction is converted into heat.

Where we WANT friction

Friction makes many useful things possible:

  • Walking: your shoes grip the ground via friction. On ice or polished floors, you slip.
  • Writing: a pencil makes marks because friction between the graphite and paper rubs off tiny bits.
  • Driving: car tyres grip the road by friction. In rain or snow, friction drops and stopping distance grows.
  • Brakes: pads press against discs to slow the car. All braking comes down to friction.
  • Climbing rope: hands and feet grip by friction.
  • Strike a match: friction between the matchhead and the rough strip on the box generates heat that ignites the chemicals.
  • Knots: rope stays tied because friction between the strands resists slipping.
  • Velcro and gecko feet: tiny hooks or hairs grip via friction.

Where we want LESS friction

Sometimes friction is wasteful:

  • Engines: moving parts in an engine waste energy to friction. Engine oil reduces it.
  • Bicycle chains: regularly oiled to keep them moving smoothly.
  • Door hinges: a bit of oil prevents squeaking.
  • Wheels and bearings: ball or roller bearings dramatically reduce friction on rotating shafts.
  • Skis: waxed to slide more easily on snow.
  • Aircraft: streamlined shapes minimise air friction.
  • Ships: smooth hulls and special paint reduce water friction.

What affects friction strength?

  • Roughness: rough surfaces have more friction than smooth ones.
  • Pressing force: more weight or pushing pressure gives more friction. (A heavy box is harder to slide than a light one.)
  • Material: rubber on dry tarmac has very high friction. Steel on ice has very little.
  • Lubrication: oil, grease, water and other fluids fill in between the surfaces, separating them and lowering friction.

Strangely, the area of contact (how big the surfaces in contact are) does NOT directly affect the size of friction. Two bricks on top of each other have the same friction whether they meet on the wide face or the narrow side.

Did you know? Without friction, you could not walk or even stand up. On a frictionless surface, you would slide instantly to your knees and then keep sliding wherever you happened to be pushed. A roundabout for children with frictionless wheels would never slow down. Your school books would slither off any desk that was not perfectly flat. Friction is one of the most useful forces in your daily life.

Lubrication

Lubricants are substances that reduce friction by getting between two surfaces. Most are liquids:

  • Engine oil: protects the moving parts of car engines, allowing them to run for years without seizing up.
  • Grease: thicker than oil, used in bearings and slow-moving joints.
  • Water: a natural lubricant. Wet roads are slippery for the same reason.
  • Graphite powder: a solid lubricant used in locks and some old machinery.
  • PTFE (teflon): one of the slipperiest known materials, used on non-stick pans, bearings and even some surgical implants.
  • Synovial fluid: your own bodys lubricant, found in joints, lets your knees, elbows and shoulders move smoothly.
Try this Place a book flat on a table. Push it gently and feel how much force it takes to start sliding. Now place a few small marbles or pencils under the book and push again: it slides much more easily, because youve turned sliding friction into rolling friction. This is the same trick the ancient Egyptians used to move giant stones for the pyramids: they placed logs underneath and rolled the stones over the logs.
Deeper dive: how engineers designed brakes that survive 100 stops per kilometre

Modern car and aircraft brakes are masterpieces of friction engineering. They need to be powerful (stopping a 2-tonne car in seconds), reliable (never failing) and able to handle the heat that friction creates.

A typical car has disc brakes on its front wheels: a metal disc attached to each wheel, with a calliper holding two brake pads on either side. When you push the brake pedal, hydraulic pressure forces the pads against the disc. Friction between the pads and disc converts the cars motion into heat and stops the car.

A heavy car decelerating quickly from motorway speed can generate over 1 megawatt of heat in the brakes for a few seconds. The brake discs can briefly glow cherry-red. Engineers use cast iron or special carbon-ceramic composites that can survive the temperatures without warping.

Aircraft brakes are even more extreme. A jumbo jet weighs hundreds of tonnes and lands at over 250 km/h. The brakes have to stop it within the runway length, generating enormous amounts of heat. Modern airliner brakes use stacks of carbon-carbon discs that can survive temperatures over 1,500 degrees Celsius without melting. Sometimes the brakes get so hot that the planes wheels have to be cooled with fans before takeoff.

Formula 1 race cars push brake technology to its limit. F1 brake discs (made of carbon-carbon) reach 1,000 degrees Celsius in normal racing and can lose 10 per cent of their mass during a single race as the material is worn away. The friction also turns the brake pads themselves into glowing dust within laps. Yet F1 brakes routinely decelerate cars from 300 km/h to standing in under 3 seconds, withstanding forces well over 5g on the pilot.

Behind every brake is the same simple physics: rough materials pressed together convert motion into heat through friction. From the wooden brake blocks on a Victorian wagon to the carbon-carbon discs on a fighter jet, the principle has not changed in over 200 years.

For more, see air resistance and what is a force.