Question

Which of the following is not a zero-effort situation?

• A. The work done by the tension in the string, when a body tied with the string move along a circular path.
• B. When a horse pulls a cart on a level road.
• C. The work is done by a coolie when he travels on a horizontal platform, with a load on his head.
• D. When a person holds some amount of load but, does not move from his position.

Answer 1

The correct option is B

When a horse pulls a cart on a level road.

The explanation for the correct option:

In the case of option B,

1. When force and displacement are perpendicular to one another and when force or displacement is zero, it is called zero work.
2. If the force and displacement are $F,d$ respectively then the work done $\left(W\right)$ is, $W=Fd\cos \left(\theta \right)$.
3. Here, $\theta$ is the angle between the force and the displacement.

For the zero work $\theta$ would be ${90}^{°}$. So,

$$\begin{gathered} W=F\times d\times \cos \left(90\right) \\ W=0 \end{gathered}$$

The explanation for the incorrect options:

In the case of option A,

1. The force in the string is a tension force that gives the circular motion with the appropriate acceleration.
2. This is why, in this context, it is also referred to as a centripetal force.

In the case of option C,

1. With bags on his head, a coolie travels on a horizontal platform.
2. On the suitcase, the coolie has done nothing. A coolie traveling on a frictionless horizontal platform while carrying a load on his head does not do any effort.

In the case of option D,

1. Even though force is applied to the bundle, the person does not move from his location (no displacement).
2. As a result, there is no work.

Hence, option (B) is the correct answer.