What is friction and why does it happen?
Friction is a force between two surfaces that touch. It acts along the surfaces and opposes sliding. If a box moves right, friction on it points left.
Why? No surface is perfectly smooth. Under a magnifying glass, even polished wood has tiny hills and valleys. When two surfaces touch, these bumps lock into each other. To slide, you must break or ride over them. Very close surfaces also stick a little, because their particles attract each other.
Friction is measured in newtons (N), like every force. You can measure it with a spring balance: pull a block at a steady speed, and the reading equals the sliding friction.
Static, sliding and rolling friction
Static friction
When an object is still and you push it gently, it does not move. Static friction grows to match your push exactly. Push 5 N, friction is 5 N. Push 7 N, friction is 7 N. But it has a limit (the maximum static friction).
Sliding (kinetic) friction
Once your push beats the limit, the object starts to slide. Now sliding friction acts. It is a little smaller than the limit. That is why it is harder to start pushing a heavy cupboard than to keep it moving.
Rolling friction
When something rolls (a wheel, a ball), the bumps do not have to slide over each other. Rolling friction is much smaller than sliding friction. Order: static (limit) > sliding > rolling.
Fluid friction (drag)
Air and water also resist motion. This is called drag. It grows with speed. Fast cars, aeroplanes, fish and birds have streamlined shapes to reduce drag.
What does friction depend on?
- Roughness of the surfaces: rougher surfaces give more friction. Sandpaper > wood > ice.
- How hard the surfaces are pressed together (the normal force): a heavier object gives more friction. Double the weight, and sliding friction roughly doubles.
- Not much on contact area: a brick on its wide face or on its narrow side gives about the same friction, because the same weight presses down.
- Not on speed (for slow sliding on solids).
For older students: sliding friction ≈ μ × N, where N is the normal force and μ (mu) is a number for the pair of surfaces.
Increasing and reducing friction: a necessary evil
Friction is useful
Without friction we could not walk, hold a pen, light a matchstick, tie a knot or stop a bicycle. Nails and screws hold because of friction.
Friction is a nuisance
It wastes energy as heat (rub your hands), wears out shoes, tyres and machine parts, and makes machines work harder.
To increase friction
- Treads on tyres and shoe soles; spikes on sports shoes.
- Sand on icy roads; chalk powder on a gymnast's hands.
- Brake pads pressed hard on the wheel.
To reduce friction
- Lubricants: oil, grease, powder on a carrom board.
- Polishing surfaces smooth.
- Wheels and ball bearings (sliding becomes rolling).
- Streamlined shapes to cut air and water drag; an air cushion (hovercraft).
Because we need it but also lose energy to it, friction is called a necessary evil. In nature, animal paws have rough pads for grip, while fish are slimy and streamlined to slip through water.
Key formulas and definitions
- Friction acts opposite to motion (or attempted motion)
- Static friction = push, up to a maximum
- Sliding friction ≈ μ × N (N = normal force)
- Static (max) > sliding > rolling
- Unit: newton (N)
Worked examples
1. A box sits on the floor. You push it with 20 N but it does not move. What is the friction on it?
The box stays still, so forces balance. Static friction = 20 N, pointing opposite to your push.
2. A block is pulled at steady speed with a spring balance reading 6 N. What is the sliding friction?
Steady speed means balanced forces. Sliding friction = 6 N, opposite to the pull.
3. Friction on a 2 kg block is 6 N. Three identical blocks are stacked (6 kg) on the same surface. Estimate the friction.
Friction is roughly proportional to the weight pressing down. Weight is 3 times, so friction ≈ 3 × 6 = 18 N.
4. Sliding friction needs 8 N to keep a box moving. You push with 14 N. What is the net force?
Net force = 14 − 8 = 6 N in the direction of the push, so the box speeds up.
Common mistakes
- Thinking friction always acts backwards to you. It acts against the sliding of the surfaces. When you walk, friction on your foot points forwards.
- Thinking a wider contact area always means more friction. For solids, area makes very little difference.
- Thinking static friction always has its maximum value. It is only as big as needed to stop motion.
- Saying friction is only harmful. Without it we could not walk, write or brake.