What is Work:
One can calculate the work done by an airplane by using the equation: W = F x D
The work done by the force F during displacement dr is mathematically defined as
(dot product)
The magnitude 
= dS
dU = F dS cos a
= (f cos a ) dS
= f t ds
= F cos a
Thus, the work can also be defined as the product of the force and the displacement in the direction of force or the product of the component of the force in the direction of displacement and the displacement. Note that F n force does no work because the angle between ds and F; is 90° . As cos 90° = 0 so F n cos a. x ds
If the force F1 is in the same direction of the displacement, the work done is positive and if F1 acts opposite to the direction of displacement the work done is negative.
The unit (S.I.) of work is N-m or Joule. The unit J Goule) is applied to work and energy only and not to moment of a force which is also N-m.
In general,
And = area under the curve
Ft – S
Ul-2 is the work done on the body, so Ul-2 is negative.
Again,
If you apply a force over a given distance – you have done work. Work = Change in Energy. If an object’s kinetic energy or gravitational potential energy changes, then work is done. The force can act in the same direction of motion. Or, the force can act against the motion. (Drag and friction do that.) Forces can act when objects touch.
In general, the energy transferred depends on the amount of force and the distance over which that force is exerted.
If the man pushes the rock in the direction of the force, he has done work. If the rock rolls back and pushes him, then the rock does work on the man.
