Question

A person holds a bundle of hay over his head for 30 minutes and gets tired. Has he done some work or not? Justify your answer.

Answer 1

When a person holds a bundle of hay over his head for 30 minutes, he applies force in the upward direction, against the gravity, so displacement is in the direction of applied force. Hence angle between force and displacement is zero.
Here, $F=mg$ and $S=h$ (height to which the person holds the hay)
we know,
$W = F×S\ cos\ θ$
$W= F×S\ cos\ 0°$
$W= F×S$
$W= mg×h$
$W = mgh$