Question

No work is done by a person moving on a road while carrying box on his head. Justify?

Answer 1

because work done is is at an angle 90° to the direction
since work is F . s cos 90°
→ cos 90°is 0(zero).
work done becomes zero.

Answer 2

When the man moves on the road with a heavy box on his head he exerts a vertically upward force on the box .But the displacement of the load is in the horizontal direction.The box has not moved any distance in the vertical direction and hence the work done by the force exerted by the man is zero.The man does no work on the box when he moves on the road.

But when the man climbs up the stairs he does positive work on the box as there is upward displacement.When he comes down the stairs he does negative work on the box .This is because the box is displaced in the vertically downward direction whereas the force exerted by the man on the box is in the upward direction.