Q. WHAT DO YOU MEAN BY POSITIVE, NEGATIVE AND ZERO WORK DONE. GIVE AT LEAST ONE EXAMPLE OF EACH CASE.

1. When θ is acute (<90), cosθ is positive. Hence work done is positive.

example: when a body falls freely under the action of gravity. 

cos0 = 1. therefore work done by the gravity on the body falling freely is +ve. 

2. When θ is acute (>90), cosθ is -ve. Hence work done is -ve.

example: when a body is thrown up, its motion is opposed by the gravity. the angle θ b/w gravitational force and displacement is 180. cos 180 = -1,  therefore work done by the gravity on a body moving upward is -ve.

3.

 when, θ = 90

cos90 = 0

work done is said to be zero. 

example: when a body tied to one end of a string is rotating in a circle, work done by the centripetal force applied along the string is zero because 

 θ = 90

cos90 = 0