Hi all, While thinking about potential energy and force, I couldn't figure out how to maintain an object at a constant height above the ground i.e to maintain a constant potential energy. I know that potential energy will be equal to the amount of workdone to lift the object to a particular height (considering gravity) but W = F x displacement. But to maintain an object at a constant potential energy (height), displacement = 0 & hence no work or energy is required. If that is right, why does helicopters expend energy (fuel), to stay at a particular height? Also consider an object on ground, what is the exact action taking place if the same magnitude of force as that of gravity is applied in the upward direction?