Question

Suppose I'm carrying a heavy bag across a horizontal plane. Why is work done? Why does it depend on the weight of the bag if no work is done?

No work is done against gravity (`W = F.S cos theta`, which I know is zero because force and displacement are perpendicular) but I still feel much more tired than if I had not carried the bag. Why?

Answer 1

That is the reason,why you would prefer a trolly to carry it.

Well you are not pushing nor pulling it over the surface,rather you have it in your hand to carry,so you are constantly supplying that amount of potential energy required for the bag to be at that height above the ground.

Your muscles are working against gravity and energy is being utilized.