Lift does not come for free, and it can be theoretically proven that, whenever lift is created by means of a wing (or indeed by any other geometry moving through unconfined fluid), a '''drag''' force will also be created. Drag is a force that constantly does negative work on an aeroplane thereby consuming its mechanical energy, which must be balanced by using either an engine (in case of a powered aeroplane) or by releasing gravitational potential (height, in case of a glider). This is why gliders must always descend ''relative to the surrounding air '' to maintain its airspeed. To keep the argument rigorous, it is easier to understand why gliders descend ''relative to air'' by following a force equilibrium argument (''See also: [[How Gliders Fly]]''). In this process, gravitational potential may be lost (usually) or gained (in case of soaring). When a glider soars, the energy it harvests from the air flow is greater than the rate of dissipation due to drag, thereby allowing the glider to climb without losing airspeed.