Question

when is the work done by a body said to be negative

Answer 1

HIi again

Work is vector i.e. along with magnitude direction also plays a vital role in determining it’s value and it could be positive, negative or even zero.

Considering the mathematical formula for Work = F.d(both vectors) it is said to be

(a) NEGATIVE when displacement of the body is in direction opposite to direction of force e.g. when a person lifts a weight then the displacement of the body is in direction opposite to Gravitational Force and thus it can be said that the work done on the body by gravity is NEGATIVE (angle between Gravitational Force and Displacement is 180 degree and cos 180 = -1).

{b) On the contrary ZERO work is said to be done when NET DISPLACEMENT of body = 0. It can happen when

net force on the body is zeroSame starting and terminating position of the bodydisplacement of body is in direction perpendicular to the force.
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Answer 2

when a body is made to slide over a rough surface ,the work done by electrostatic force is negative.

when brakes are applied on a moving vechile the work done by breaking force is negative