Question

In which case, work is not done?
$\left( {\text{A}} \right)$ A girl swimming in pond
$\left( {\text{B}} \right)$ Windmill lifting water from a well
$\left( {\text{C}} \right)$ A standing man holding a suitcase in his hand
$\left( {\text{D}} \right)$ A sail boat moving in the direction of wind

Answer 1

Here first we discuss the definition of work.
That is, Work is defined as the product of force and displacement in the direction of the force. If there is no displacement in the direction of applied force, work done will be zero.
Then we discuss the option one by one and then we conclude the required answer.

Complete answer:
Option A: Here, a girl is swimming in the pond.

The girl is represented by a block of mass ${m_1}$. The free body diagram of the girl is as shown above. As we can see that the motion of the girl is in the direction of net applied force F. The vertical forces balance each other for a floating body. Now, since the displacement is in the direction of the applied force, we say that there is work done by the girl. Hence, Option A is not correct.
Option B: In this case, the windmill is lifting water from a well. So in this case the bucket which will be carrying water will be moving up and the windmill is applying a force in the upward direction to lift this bucket. Since there is displacement in the direction of the applied force, we say that work is done here. Hence option B is also not correct.
Option C: In this case, a man is holding a suitcase. The man will be applying a force in the upward direction to hold the suitcase, but the suitcase will be stationary, i.e. there is no displacement in the direction of applied force. Hence, the work done is zero. So, option C is correct.
Option D: A sailboat is moving in the direction of wind. So, there is displacement as well as the force applied by the wind. Hence there will be the work done. So, option D is not correct.

Hence the correct option is (C).

Note: Do not confuse work with energy. In option C, the man is spending energy to hold the suitcase, but the work done is zero as displacement is zero. But, if it is asked that the man is lifting the suitcase, then there will be work done.