Question

In which of the following cases is the work done positive zero or negative?
(A) Work done by the partner on a suitcase in lifting it from the platform on to his head.
(B) Work done by the force of gravity on the suitcase as the suitcase falls from the partner's head.
(C) Work done by the porter standing on a platform with a suitcase on his head.
(D) Work done by the force of gravity on a ball thrown up vertically up into the sky.
(E) Work done by force applied by hands of a man swimming in a pond.

Answer 1

Hint : Here we need to understand the work done formula, here we now that work done is zero when only force is applied by a body without any displacement, positive when the displacement is in the same direction in which force is acting, and negative when the displacement is in opposite direction of force applied.
$work = force \times displacement \times \cos \theta$
Here theta that is the angle of the cosine function represent the angle between the force and displacement.

Complete Step By Step Answer:
Here in the first option we see that angle between the force applied and displacement is ninety degree hence,
$\Rightarrow work = force \times displacement \times \cos 90 = force \times displacement \times 0 = 0$
Now in the second option the work done is positive because force applied by the gravity and the displacement are in the same direction.
In the third option work done is zero because there is no displacement.
Now in the fourth option the work done is negative because force applied by the gravity and the displacement are in the opposite direction.
Now in the last option the force applied and the displacement are in the same direction hence the work done is positive.

Note :
In order to solve the question related with work done we should take care of the direction of the quantities because work done is a vector quantity and for solving vector quantities we should take care of the direction of the quantities related.