Q.State any four methods to reduce friction.

Friction is the tangential force that opposes relative motion (or impending relative motion) whenever the surfaces of two bodies are in contact and one slides, or tends to slide, over the other. Friction is not simply mechanical interlocking of the microscopic bumps and grooves every real surface has — if it were, polishing a surface smoother (removing those irregularities) should reduce friction, but in practice friction does not decrease with polishing, and can even increase, ruling out the interlocking picture.

The modern explanation is molecular attraction. Because real surfaces are irregular at a microscopic scale, the true (actual) area of contact between two pressed-together surfaces is far smaller than their visible, apparent area of contact — the surfaces genuinely touch only at a relatively small number of high points. Because this true contact area is so small, the pressure at those contact points is very high, producing a strong molecular force of attraction between the two surfaces exactly there — called a cohesive force if both surfaces are the same material, or an adhesive force if they are different materials. As a surface is polished smoother, its true contact area actually increases (fewer gaps between high points), which increases this molecular attraction and hence increases friction — the opposite of what the simple interlocking picture would predict, but exactly consistent with what is observed. This is also why lubricants (a different material such as oil or grease placed between the two original surfaces) reduce friction: they physically separate the true contact points and weaken this attraction.

Friction is deliberately reduced, when it is a nuisance (dissipating energy as heat in machines, lowering efficiency), using lubricants or by converting sliding friction into rolling friction with ball bearings. But friction is also essential in everyday life: it lets us walk (friction between feet and ground), hold objects in our hands (static friction), brake vehicles, and climb trees — without it, none of these ordinary actions would be possible.