Question

A block of mass 10 kg rests on a rough inclined plane making an angle of 30° with the horizontal. The coefficient of static friction between the block and the plane is 0.7. The frictional force on the block is (g = 10 m/s²)

(1) 50 N

(2) 20 N

(3) 30 N

(4) 70 N

Answer 1

(1) 50 N

The F.B.D. of block is shown in Fig. obviously N = mg cos 30°

As μ_s =  0.7

So maximum force of static friction.

Force down the plane on mass m = 100 sin 30

= 50 N

Since the force trying to move block is less than f_lt ; the block does not move. The friction coming into play equals the force trying to move the block i.e. force of friction = 50 N.