Answers

2015-05-20T12:32:17+05:30
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

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2015-05-20T17:42:17+05:30
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
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