Friction, Types and Effects

Every time you write with a pencil, something is grinding pencil graphite onto paper. That's friction, and without it, your pencil would slide right off the page.

Friction is a contact force that opposes motion, or attempted motion, between two surfaces. A simple model of surface roughness helps explain it, although adhesion between materials also contributes.

No surface is truly smooth. A tabletop that feels perfectly flat under your hand is, seen closely enough, a landscape of ridges and pits. When two surfaces sit together only the high points really touch, and those high points settle into each other's dips like two sheets of very coarse sandpaper laid face to face. To slide one surface across the other you have to lift, bend or shear off those interlocking high points, and the resistance you feel while doing it is friction.

Three things follow straight from that picture, and none of them has to be memorised on its own. First, friction opposes relative sliding, or the tendency to slide, at the contact, because the high points only resist when you try to drag them past one another; push a box east and friction pulls west. Second, the two things that change how much friction you get are exactly the two things that change the high points. Rougher surfaces have larger and more numerous points to catch on, so they grip harder. A larger normal force, meaning the surfaces are pressed together more firmly, drives the points deeper into each other so more of them interlock, which is why a loaded box is harder to drag than an empty one across the very same floor. Third, all the work spent bending and shearing those points has to go somewhere, and it goes into making the surface atoms vibrate faster, which is heat. The warmth you feel when you rub your hands together is the energy of your motion being converted, not disappearing.

Keep the direction and the size apart, because that is where this most often goes wrong. The direction of friction depends on how the surfaces slide, or would slide, relative to one another. It need not oppose the motion of the whole object: static friction can push a driven car tyre forwards. What sets the size depends on whether the box is already sliding, and the two cases are different enough to be worth separating. While the box is still stationary, friction matches whatever you push with: lean on it with 20 N and the high points grip back with 20 N, so the net force is zero and nothing moves; push with 50 N and they answer with 50 N. That is what the definition means by opposing attempted motion, and it is why a box can sit completely still under a hard push. But the high points can only grip so far. Once your push passes the hardest grip they can manage, they shear and the box breaks away into a slide, and from then on the friction stays roughly the same size however much harder you push. Pushing a sliding box harder does not make the floor rougher, so the extra push is left over to speed the box up.

Static friction adjusts to the force needed to prevent sliding, from zero up to a maximum. For the same surfaces and normal force, this maximum static friction is usually greater than sliding friction. The actual static friction can be small, for example when you push a resting box gently. A larger push is usually needed to start the box sliding than to keep it sliding steadily. Comparing the maximum, rather than every possible static-friction value, explains that difference.

Rolling friction is smaller again, and for a different reason rather than more of the same one. A wheel does not drag its surface across the ground at all. It touches at one small patch, and that patch is peeled up and set down rather than scraped along, so the high points never have to be sheared past each other. This is the whole reason the wheel was worth inventing: putting a heavy box on wheels does not make it lighter and does not make the floor smoother, it changes which kind of friction you are fighting.

Once you know that friction comes from interlocking high points, controlling it becomes obvious in both directions. To reduce friction you keep the high points apart: a lubricant such as oil or wax fills the dips so the surfaces float on a liquid layer instead of touching, ball bearings swap sliding for rolling, and streamlining reduces how much fluid a moving object has to shoulder aside. To increase friction you give the high points more to grip: grip tape and the cleats on a football boot help surfaces resist slipping. Tyre tread grooves have a different role on wet roads: they channel water away so the rubber can contact the road.

Friction is not a good force or a bad force. It is the same interlocking of surface high points in every case, and whether we call it useful or harmful depends only on whether we wanted that motion to happen. A brake pad and an engine piston are doing physically identical things, grinding two surfaces together and turning motion into heat. We pay a lot of money for the first and a lot of money to prevent the second. Read the two lists below with that in mind, and notice that engineers are almost never trying to remove friction, only to move it to where it helps.

Useful friction, we'd be helpless without it:

• Car tyres gripping the road at the Bathurst 1000 (Mount Panorama), where tyre compound choice is critical. Hard tyres last longer but give less grip; soft tyres give more grip but wear out faster. The reason is worth knowing, because rubber is a special case. Most materials are stiff, so pressing them harder does not change how much of them genuinely touches. Rubber is soft enough to squash down into the road's high points and mould around them, so a softer compound puts more rubber in real contact and grips harder. The price is that the same softness lets more rubber be torn away each lap, which is why the grippiest tyre is also the one that wears out first, and why engineers balance the two for every stint.
• Safety boots on construction sites Australian workplace safety requires grip
• Writing and drawing pencil graphite needs friction to stick to paper
• Brakes brake pads press against discs, sliding friction converts kinetic energy to heat, slowing the car
• Climbing ropes friction between the rope and rock anchor holds a climber's weight

Harmful friction, wastes energy and causes wear:

• Engine wear moving metal parts grind against each other; engine oil is essential to reduce this
• Brake pad wear brake pads gradually erode away (the energy goes into heat and sound)
• Energy loss in machines about 20% of fuel energy in a car is lost to friction in the engine and drivetrain
• Bushfire risk sparks from machinery friction can ignite dry Australian bush; fire risk protocols include checking for friction-generating faults

In each set below, three share the same type of friction and one does not. Identify the odd one out and explain why.

Set 1: A book sitting still on a tilted board · A car parked on a hill · A foot not yet sliding on a wet floor · A ball rolling down a ramp

Set 2: A swimmer in a pool · A skydiver falling · A car driving through rain · A book sliding on a desk

Set 3: Engine oil lubricating pistons · Glide wax on skis · Adding grip tape · Grease on a bicycle chain

Set 4: A hockey puck sliding on ice · A book sliding off a table · A ball rolling across carpet · Chalk scraping a blackboard

Design a fair test to compare friction on 3 different surfaces (e.g. wood, sandpaper, carpet). Your plan must include:

• Aim: What question are you trying to answer?
• Independent variable (IV): What one thing are you changing?
• Dependent variable (DV): What are you measuring?
• Controlled variables (at least 2): What stays the same to make the test fair?
• Method: Briefly describe the steps
• Expected result: Which surface do you predict has the most friction?