Question

A block of mass 20 kg is being pulled by a force F on a rough horizontal surface. If the

coefficient of friction is 0.4, calculate
a) the normal force, N
b) the static frictional force, f
c) the minimum force required for the block to move with uniform speed​

Answer 1

The normal force counteracts any vertical force acting on the system and is perpendicular to the surface. So find the weight of the block and add the two forces.

N - W = 0 because we aren’t accelerating in y direction.
W = mg = 20*9.8 = 196 Newtons
N -196 = 0
N = 196

The definition for frictional force is the normal force multiplied by the coefficient. Very simple formula.
N * us = Fs
196*0.4 = 78.4 Newtons

You kind of already found the minimum force. You would need to apply a force larger than 78.4
F - 78.4 = 0 because we are moving at a constant uniform speed
F = 78.4