Give examples to show that friction is both a friend and a foe.

Explanation

Friction is both friend and foe that is why it is said that friction is a necessary evil.
(i) Frietion as a friend :
(a) To hold a glass, we have ridges on our palm, which increase the friction between palm and glass.
(b) We do write anything with pen or pencil because there is friction between the surfaces of paper and point of pen (nib) or pencil.
(c) Teacher writes on black-board with chalk because of friction between black-board surface and the chalk.
(d) If there is no friction, then a moving body would never stop.
(e) Had there been no friction between the tyres of the automobiles and the road, they could not be started or stopped or turned to change the direction of motion.
(f) A nail could not be fixed in the wall or a knot tied had there been no friction. (g) No building could be constructed without friction.
(ii) Friction as a foe:
(a) Friction wears out materials whether they are screws, ball-bearings or soles of shoes.
(b) Friction can also produce heat which increases wear and tear of machine parts. It also causes much wastage of energy because this heat is not utilized.