Discuss whether or not work is done in the following cases:
(i) When we twist a wire.
(ii) When we press a football.
(iii) When we push a table.
(iv) When a person carries a hand bag and walks on a level road.
(v) When a person holds a book in his hand and keeps it stationary.
(i) Twisting a wire changes it's shape. So, work is done against internal forces of cohesion between the molecules of the wire.
(ii) The volume of a football changes when we press it. So, work is done against the pressure of the gas inside the bladder of the football.
(iii) Work is done against the force of friction between the legs of the table and the floor, when we push a table between the legs of the table and the floor.
(iv) No work is done in this case. The force the person is applying against the weight of the bag is acting vertically upwards and his motion is in the horizontal direction.Thus, no motion in the direction of the force is applied.
(v) In this case, though the force is constantly being applied, there is no displacement and hence work done is zero.
