Question

a body of mass 20kg is placed on a horizontal surface whose cofficient of friction is 0.5. find the frictional force acting on it (g=10ms-2)

Answer 1

mass of the body = 20kg
so normal reaction acting on the body = 20×g = 200 N
but if the body is not pushed there will be no frictional force acting on it 
if the body is pushed there will b a frictional force opposing the motion of the body
& if the pushing force is gradually increased then the static friction will be increasing & reach a maximum of μ×200 N = 100 N .(limiting friction)
when the body starts moving it will experience a kinetic friction which is less than the limiting friction

Answer 2

Mass = 20 kg
Coefficient of Friction = 0.5
Normal Force = Mass x g = 20x10 = 200N
Force of Friction = Coefficient of Friction x Normal or Perpendicular Force
                           = 0.5x200
                           = 100N