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Work in the scientific parlance is different from the colloquial parlance. You may strain yourself a lot pushing against a very heavy steel cupboard. Would you be successful in displacing it? You may not have been successful in doing work on the cupboard since it did not move. In the scientific sense work is done only when the object on which force is exerted moves in the direction of the force.

A porter lifts a suitcase and walks down the platform in a railway station; we hit a nail into a wall; an elephant lifts a huge log of wood and carries it to the bank of a river, etc. We can say that work is done in these cases, whereas when we sit and read a book or wait at the bus stop for a long time we are not doing any work.

"Work is said to be done when a force acts on a body and the point of application of the force moves through a certain distance"



Two conditions need to be satisfied for work to be done:

i) a force should act on an object and

ii) the object must be displaced.


If any one of these conditions is not satisfied, work is not done.

When a force F acts on an object and displaces it through a distance 'S' in the direction of the force,

Work done W = Force x displacement = F x S (Newton metre) or joule.

W = FScosθjoule or

W = FS
cosθ J.

The S.I. unit of work is joule (J). Work has only magnitude. Hence it is a scalar quantity.

1. 'One Joule is the amount of work done when a force of 1N acts on an object and displaces it by a distance 1 m along the line of action of the force'.

Consider a child pulling a toy car along the ground. The force exerted on the toy, and the displacement of the toy is in the same direction. In this case we say that the work done is positive.

Consider a porter lifting a suitcase to his head. He is applying a lifting force on the suitcase and the displacement is also in the direction of the lifting force. But then there is the gravitational force also which is acting on the suitcase, but in the opposite direction. The angle between the line of force of gravity and the displacement is 180°. In such cases the work done is taken as negative.

Consider a rope in which one end is tied to a lamp post and the other end is in your hand. If you pull the rope, the lamp post will not get disturbed rather you will feel that you are moving in the forward direction. Here the applied force ‘F’ and the displacement ‘S’ are perfectly in the opposite direction. Hence the work done is negative.

When a force acts at right angles to the direction of motion (direction of displacement), work done is said to be zero. Similarly work done on an object moving in a circular path is also zero. This is because the centripetal force that keeps the object in the circular path is acting along the radius of the circle and is at right angles to the direction of motion.


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