Question
No work is done by a man moving on a horizontal road while carrying a box on his head. Explain.

Answer

The displacement of the man and box is along the horizontal direction. Thus, the angle between the displacement and the force of gravity is $90^0;$
Thus, W = Fd $\cos 90^0 = 0$
Hence, no work is done against gravity in this case; however some work is done against friction.

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