Positive Work, Negative Work and Zero Work

Equation of Work: 

Mathematically work is expressed by the following equation. 

W = F . S . cos θ 

F = force, S = displacement, θ is the angle between force vector and displacement vector. 

There are three situation for work done:

Positive Work: Work done is positive when the force is in the
direction of displacement.

Example of positive work: 

The baby has exerted a force in the direction of displacement of the car. In this situation, the work done will be equal to the product of the force and displacement. In such situations, the work done by the force is taken as positive.

Zero Work: When after applying an amount of force but there is no displacement. Then the angle between these two vectors is 90^o. 

Example: A large and heavy rock is moved by applying force but no displacement noticed, in this case work will be calculate zero. 

Negative Work: Work done is negative when the force acts
opposite to the direction of displacement.

Consider a situation in which an object is being displaced by the action of forces and we identify one of the forces, F acting opposite to the direction of the displacement s, that is, the angle between the two directions is 180º. In such a situation, the work done by the force, F is taken as negative and denoted by the minus sign. The work done by the force is F × (–s) or (–F × s).