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Let us consider some cases where work is not done:

  • Work is zero if applied force is zero (W=0 if F=0): If a block is moving on a smooth horizontal surface (frictionless), no work will be done. Note that the block may have large displacement but no work gets done.
  • Work is zero if Cos θ is zero or θ = Π/2. this explains why no work is done by the porter in carrying the load. As the porter carries the load by lifting it upwards and the moving forward it is obvious the angle between the force applied by the porter and the displacement is 90o.
  • Work done is zero when displacement is zero. This happens when a man pushes a wall. There is no displacement of the wall. Thus, there is no work done. Similarly when a car is moving on a road, there will be a frictional force applied by the road on the. There will be work done by the frictional force on the car. What is the work done on the road by the car? From Newtons third law we can say that to every action, there is an equal and opposite reaction. Thus the force applied by the road on the car will be equal and opposite to the force applied by the car on the road. Since, there is no displacement of the road, there  will be no work done on the road.