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

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

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