Work

Work Done

Work is done only when the object moves in the direction of force and is given as the product of the force and the distance moved by the object in the direction of the force.

                 W = F * s
where W = work done (in J)
             F = force (in N)
             s = distance moved in the direction of the force (in m)

SI Unit: J
Scalar quantity

One joule is defined as the work done by a force of 1 N which moves an object through a distance of 1 m in the direction of the force.

NO work is done when
(1) Object does not move
(2) Direction of force is perpendicular to the direction of motion of object
work done
Courtesy of HyperPhysics
Note that if there is more than one force acting on an object, we have to decide which force is used to calculate the work done. 
Example:

Figure shows a block being pushed up a slope from position A to position B. Calculate
(a) work done by F,
(b) work done against friction.
(c) useful work done
Answer:
(a) W.D by F = F x s = 120 x 5 = 600 J
(b) W.D against friction = f x s = 30 x 5 = 150 J
(c) useful work done = 600 – 150 = 450 J

Work Done v.s Moment

Criteria Work done Moment
Definition Force x distance moved in the direction of force Force x perpendicular distance from pivot to line of action of force
Unit Joules (J) Nm
Scalar or Vector Scalar Vector
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