In Physics, work is defined in a very specific way to describe what is accomplished by the action of force on an object. Work is said to be done if the force applied on the object succeeds in moving it. If no motion takes place, no work is said to be done. For example, an engine pulling a train does work.
A person who lifts a stone does work. Work is done to push or pedal a bicycle. However, a person may exert a force and yet do not work, in the specific sense of the word, because the force may not produce any motion.
For example, a man pushing against a wall does not work if the wall does not move. Similarly, if you hold a heavy object in your hand and stay motionless, you do no work on it even though you may get tired holding it. Therefore, for work to be done, two conditions must be fulfilled.
(i) A force must be exerted and
(ii) The force must produce motion or displacement.
Only that force, which gives rise to motion, does work.
The work done by a force on a body is defined as the product of the magnitude of the force and the distance moved in the direction of force.
Work done = force x distance moved in the direction of the force.
Work done = force x distance
If the force and the displacement are at right angles to each other, no work is done on the body by that force.
Some Situations in which No Work is Done:
A man, carrying a bucket of water, walking on a level road with a uniform speed in a straight line does not work.
A boy, swinging a stone, tied to a string, with a uniform speed in a circle in a horizontal plane over his head, does not work.
Work is a scalar quantity.
Unit of Work:
Since work is the product of force and distance, the unit of work is also the product of the unit of force and the unit of distance. In the SI system of units, the unit of force is Newton (N) and the unit of distance is metre (m). Hence, the unit of work is Newton metre (Nm).