Forces · Process
Friction
Nothing about a crate changes when you start sliding it. So why is the hardest part of the whole job the first centimetre?
Start here
It will not budge, and then suddenly it slides.
You lean on a full crate and push harder and harder. Nothing. Then it gives, and once it is moving you can keep it going with a much gentler push than the one that started it.
Why is starting it harder than keeping it going?
Friction is a force between two surfaces that resists them sliding across each other, and it is at its largest just before the sliding starts. Left alone the two surfaces settle into one another; once they are sliding they never get the chance to settle again, so the friction drops. The crate did not get lighter, and your push did not get stronger.
Friction acts between two surfaces that are touching, and it always acts against the sliding — never with it. It is there before anything moves, which is why a book stays put on a sloping desk lid, and it turns movement into heat, which is why your hands warm up when you rub them.
At the bench · block and spring balance
Two readings from every drag test
Change a control to begin
A spring balance is hooked to the block and pulled horizontally. Take one reading at the instant it breaks away, and a second while it slides steadily. Change the surface. Change the load.
Commit first. The same block is dragged over carpet, then over polished wood. What happens to the reading?
The surface underneath
The load in the block
Weight of the block
—
To break it away
—
To keep it sliding
—
The gap
—
Four things friction always does
Every reading on the bench obeyed all four.
It acts against the sliding
Always the opposite way to the sliding between the two surfaces — or to the sliding that is about to happen. That is not always the opposite way to the object: when you walk, your shoe pushes backwards on the ground, so friction pushes your shoe forwards. It is what you walk on.
It depends on both surfaces
Swap the floor and the reading changes even though the block has not. Rough or smooth is a fact about the pair, not about one of them.
It grows with how hard they press
Load the block and every reading rises. On the bench, doubling the weight doubled both readings.
It turns movement into heat
Rub your hands. Feel a brake disc after a long hill. The energy does not vanish — it leaves as heat, and it is not coming back.
Wanted here
- Shoes on a wet pavement
- A brake pad on a wheel rim
- A nail staying in a wall
- A tyre turning a corner
Not wanted here
- A bicycle chain
- A drawer that sticks
- A hip joint
- A piston in an engine
Friction is not a fault in the world to be designed out. Half of it is the problem and half of it is the point: the same force that wears out a brake pad is the force that lets you walk, and a floor with no friction is a floor nobody can stand up on.
Key fact
Friction is a contact force between two surfaces that always acts against the sliding. It depends on both surfaces and on how hard they are pressed together, it is largest just before sliding starts, and it turns movement into heat.
Think again
“Friction only happens once something is moving.”
Tilt a desk lid slowly with a book on it. For the first few degrees the book stays exactly where it is — and gravity is pulling it down the slope the whole time, so something must be holding it. That something is friction, acting up the slope, matching the pull along the slope newton for newton. Keep tilting and there comes an angle where friction runs out of matching to do, and the book goes. Friction between surfaces that are not sliding is doing most of the work friction does anywhere: it is what stops a ladder slipping, a screw turning back out, and a parked car rolling down a hill.
“A smooth surface has no friction.”
Two sheets of glass are about as smooth as anything you will handle, and pressed together they are notoriously hard to slide apart — smooth is not the same as slippery. Under a microscope no surface is flat: what looks polished is a landscape of peaks, and only the peaks are actually touching. Sliding means dragging those peaks over each other, and on a very smooth surface there are so many contact points that the two can stick together instead. What makes something slippery is usually a layer of something else in between — water on ice, oil in a bearing, graphite in a lock — keeping the two surfaces apart rather than making either one smoother.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Apply
A sledge is pulled across level snow at a steady speed with a 40 N pull. What is the friction on it?
Rung 2 · The one that catches people
A heavy toolbox sits on a sloping garage roof and does not slide. Which statement is right?
Rung 3 · Explain
A cyclist oils the chain and fits new brake blocks on the same afternoon. Explain why one job is about reducing friction and the other is about increasing it, and why both make the bicycle better.
Rung 4 · Take it somewhere new
In curling, two players sweep the ice hard in front of a sliding stone. Sweeping melts a very thin film of water on the surface. Explain what this does to the forces on the stone and why it travels further.
Key note
Friction is a contact force between two surfaces, acting against the sliding. Its size depends on what the two surfaces are and how hard they are pressed together, and it is bigger just before sliding starts than during the slide. It turns movement into heat, so it is wanted where grip matters and reduced — by lubricating, by rollers, by wheels — where it only wastes energy.
Going further
Press two flat metal blocks together and they touch far less than they appear to. Real surfaces are hills and valleys, so contact happens only at the highest peaks — and the true area in contact can be a tiny fraction of the area you can see and measure with a ruler. Push harder and those peaks flatten slightly, bringing more of them into contact, which is the reason friction grows with how hard the surfaces are pressed together rather than with how big the block looks. At the points that do touch, atoms of one surface come close enough to bond weakly to atoms of the other, and sliding means breaking those bonds and making new ones, over and over. That is where the heat comes from, and it is why the crate is hardest to start: given a moment at rest, the peaks settle deeper and more of those bonds form.
Because friction converts movement into heat, and heat spreads out and is difficult to use for anything, it is the reason no machine gives back everything you put in. Estimates of how much of the world's energy ends up wasted in rubbing contacts run to something like a fifth of the total — which is why bearings, gearbox oil and even the dimples on a golf ball are worth engineering carefully. The oldest fix is the simplest: get something between the two surfaces. Water under a hydroplaning tyre does it accidentally and dangerously; oil in a bearing does it deliberately, floating the metal apart so the peaks never meet. Your knee does it with a fluid that is more slippery than any oil humans make.
Before this lesson
Connects to
At GCSE this becomes
- Work done against friction, energy dissipated to the surroundings, and drag as one of the forces in a free-body diagram.
Where to next
Ask Mr Badmus AI
Want to test whether friction is helping or wasting in a machine of your own?
The drag bench is a teaching model. Each surface is given a fixed grip figure chosen to be typical, and the break-away reading is modelled as one fifth more than the sliding reading; real surfaces vary with polish, dust, damp and wear, and a real spring balance reading wanders as you pull. Weight is taken as mass in kilograms × 10 N/kg, and only the horizontal forces are drawn.
Lesson content © MrBadmusAI.