Mark the correct statement (s) regarding friction .

Options

A. Friction force can be zero though the contact surface is rough
B. Even though there is no relative motion between surface friction force may exist between then
C. The expressions `f_(L) = mu_(s) N or f_(k) = mu_(s)N ` are approximate expressions
D. The expression `f_(L) = mu_(s)N ` rells that the direction of `f_(L)` and `N` are the same.

Answer

a,b,c

Explanation

For (i) Consider a block at rest a rought surface and no force (horizontal) is acting on a Now friction force on it would be zero

For (ii) Consider a heavy block under the appliaction of small force F which is not sufficient to case its motion ,so friction force is static in nature and block does not move
For (iii) Refer theory
For friction force and normal force always act perpendicular to each after