Question

A body is being raised to a height h from the surface of earth. What is the sign of work done by :

(i) Applied force

(ii) Gravitational force?

• A. Positive, Positive.
• B. Positive, Negative
• C. Negative, Positive
• D. Negative, Negative

Answer 1

Answer: (B)

Positive, Negative

Answer 2

Work done, $\mathrm { W } = \overrightarrow { \mathrm { F } } \cdot \overrightarrow { \mathrm { d } } = \mathrm { Fd } \cos \theta$

Where $\theta$ is the angle between the force and displacement $\overrightarrow { \mathrm { d } }$

(i) As the body moves along the direction of applied force.

$\therefore \theta = 0 ^ { 0 }$ , W = Fdcos $0 ^ { 0 }$ =Fd. It is positive,

(ii) As the body moves in a direction opposite to the gravitational force which acts vertically downwards.

$\therefore \theta = 180 ^ { \circ }$ W = Fdcos= - Fd. It is negative.