Friction

Friction is the force between two surfaces that opposes their relative motion. Cause: tiny bumps (irregularities) on both surfaces interlock with each other — sliding them apart requires effort. Under a microscope every surface looks rough — that is why friction always exists, even on surfaces that appear smooth.

In sliding, the entire rough surface is dragged against the other surface (lots of rubbing). In rolling, the surface touches only at one point at a time (it keeps rotating) — less contact means less rubbing. That is why wheels and ball bearings are used: one push and the object rolls a long distance (cycles, cars, fans all rely on rolling).

The surface of ice is very smooth, and when pressure is applied it melts slightly, forming a thin film of water — that water acts as a lubricant — friction becomes very low — your feet slip. Sprinkle sand on it — the surface becomes rough — friction increases — you get grip. This is exactly what is done on icy roads at high altitudes.

(1) Walking — feet grip the ground (without it you would slip); (2) Brakes — stop a vehicle (without it there would be accidents); (3) Writing — pen grips paper. Additional examples: lighting a matchstick, holding nails and pins, turning a bottle cap.

Oil, grease, or powder forms a thin layer between the two surfaces — it fills in the bumps (irregularities) — the two surfaces no longer touch each other directly — rubbing is reduced — the machine runs smoothly and generates less heat. Examples: cycle chains, machine parts, door hinges — all use lubricants.

Tread refers to the ridged lines on a tyre — they channel water away from the contact area and provide extra grip (more interlocking with the road surface). A new tyre grips well; a worn (smooth) tyre becomes slippery — leading to accidents. That is why worn-out tyres must be replaced.