Question

A body is being raised to a height $h$ from the surface of earth. What is the sign of work done by applied force and gravitational force respectively?

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

Answer 1

Answer: (B)

Positive, Negative

Work done, $W=\overrightarrow{F}\cdot \overrightarrow{d}=Fd\cos \theta$
where $\theta$ is the angle between the force $\overrightarrow{F}$ and displacement $\overrightarrow{d}$
(i) As the body moves along the direction of applied force.
$\therefore \theta =0^{\circ },W=Fd\cos 0^{\circ }=Fd$. It is positive.
(ii) As the body moves in a direction opposite to the gravitational force which acts vertically downwards.
$\therefore \theta ={180}^{\circ },W=Fd\cos {180}^{\circ }=-Fd$. It is negative.