Question

A horizontal force of 20 N is applied on block of mass 4 kg resting on a rough horizontal table. How much fructional force the table is applying on the block.What can be said about the coefficient of static friction between the block and the table ?

Answer 1

Normal force applied by the contact surface on the block is 4g = 40 N...
the maximum frictional force that can act on the block by the table = (40 x u) N... where u is coefficient of static friction.

if the body is at rest even after applying the force of magnitude 20 N , we can say that the frictional force acting on the block is 20 N.. because if the body is at rest the magnitude of frictional force is equal to the magnitude of applied force .

so, for static friction,

40u >= 20
=> u >= 0.5

hence, coefficient of static friction is greater than or equal to 0.5 ...