Friction Notes

When one surface moves or tends to move over another, friction acts along the surfaces in the direction opposite to the motion. It arises because no surface is perfectly smooth — even polished surfaces have tiny bumps and hollows that interlock.

Friction depends on how hard the two surfaces press together (it increases when the object is heavier or pressed harder) and on the nature of the two surfaces (rougher surfaces give more friction).

Friction is a necessary evil: we could not walk, write, or hold anything without it, yet it also wears out machine parts and wastes energy as heat.

Static friction acts when a body is at rest and a force is applied but the body has not yet moved; it adjusts itself up to a maximum value. Once the body starts sliding, sliding (kinetic) friction acts, and it is slightly less than the maximum static friction — that is why it is harder to start moving a heavy box than to keep it moving.

Rolling friction acts when one body rolls over another. It is much smaller than sliding friction, which is why wheels and ball bearings are used — rolling a load is far easier than dragging it.

Order of magnitude: rolling friction < sliding friction < maximum static friction, for the same pair of surfaces.

We increase friction where we need grip: treads (grooves) on shoe soles and tyres, brake pads, and spreading sand or gravel on a slippery road.

We reduce friction where it wastes energy: lubricants (oil, grease) fill the tiny gaps and keep surfaces apart; ball bearings convert sliding into rolling; and polishing surfaces smooths them. A thin layer of air is even used in some machines to reduce friction to almost zero.

Fluids (liquids and gases) also exert a friction-like force called drag on objects moving through them. Drag increases with speed, with the area facing the flow, and with the nature of the fluid. Birds, fish, aeroplanes, and cars are given streamlined shapes so that they experience less drag.