Nature of Work Done.

Positive Work:

Work done is positive if force or its component displaces the body in the direction of force.

Work done is positive if angle between force and displacement is acute()

Example When a spring is stretched, work done by the external (stretching) force is positive.

Maximum work : W_max= Fs

When cosፀ = maximum = 1  i.e ፀ=0

It means force does maximum work when angle between force and displacement is zero.

Negative work: Work done is negative if angle between force and displacement is obtuse ()

The negative work signifies that the external force opposes the motion of the body.

Minimum work(Maximum Negative) :W_min=-Fs    When cosፀ = maximum = -1  i.e ፀ=180⁰

Zero work: It means no work is done by the force if acting.

Conditions for zero work   (W= Fs cosፀ=0)

(1) If the force is perpendicular to the displacement

W= Fscos90⁰ =0

When a person travels on a horizontal platform with a load on his head, work done against gravity by the coolie is zero.

(2) If there is no displacement (s = 0)

W= Fscosፀ = 0

Example: A man pushing a wall will result in zero displacement.

(3) If Force is zero

Example: Example: Motion of an isolated body in free space.