Q.Define 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 …