Question

Choose the one which is different from others

• A. Work done by the lifting force
• B. Work done by the gravitational force when a body is lifted
• C. Work done by the braking force of a moving car
• D. Work done by the frictional force when a body is dragged along a rough surface

Answer 1

The correct option is A

Work done by the lifting force

The explanation for the correct option:

A. work done by the lifting force:

The force of gravity is acting in the downward direction when a body is lifted but, the displacement of the body is in the upward direction. Since the angle between them is $0^{\circ }$
We know,
$$\begin{gathered} W=FS\cos \theta \\ W=FS\cos 0^{\circ } \\ W=FS \end{gathered}$$

where $F=$force acted, $S=$displacement and $\theta =$gradient between the direction of force and displacement of the body.
So, the Work done will be positive.

The explanation for the incorrect options:

B. work done by the gravitational force when a body is lifted:

The Work done by the gravitational force is negative as the gravity acts in a downward direction but the displacement is in opposite direction, that is both are in opposite directions. thus, the angle between them is ${180}^{\circ }$. So, the Work done will be negative.

C. work done by the braking force of a moving car:

When a moving vehicle applied brakes, the negative work is done by the braking force. The braking force acts in the reverse direction while the displacement of the vehicle is in the forward direction. thus, the angle between them is ${180}^{\circ }$. So, the Work done will be negative.

D. work done by the frictional force when a body is dragged along a rough surface:

The force of friction always acts in opposite direction to the motion of the body. thus, the angle between the frictional force and displacement is ${180}^{\circ }$. So, the Work done will be negative.