# Work and energy example [duplicate]

The normal definition of Work, as far as I have read, is: Energy is transferred through work. But I just heard an educationist state this example, where there is a weightlifter who keeps up a huge mass above his head (stationary), and he (the educationist) says that there is no work done, as the force exerted on the mass does not move the mass. But isn't the weightlifter doing work to keep the mass above his head? Or should I assume that there is no work done if a force exerted on an object does not move the object?

• Hi Ramana. This has been asked before. See the link I've posted above. Commented Jun 16, 2016 at 7:27

There are two kinds of energy to keep track of: potential energy and kinetic energy. Work can be thought of as the change in energy (because of the "work-energy theorem"). And the key question to ask while the weightlifter keeps his weight in the air is: is there a change in energy? Kinetic energy is a function of movement. If there's no change in movement, there's no change in kinetic energy. Since the weight remains stationary, the kinetic energy remains at where it was (zero kinetic energy since there's no movement at all). There is no change in Kinetic energy. Potential energy is a function of position. Potential energy can only change if the object's position changes. But the weight remains in the same position of space, thereby the potential energy remains the same. There is no change in energy to be found, which means no work was done.

When the weightlifter lifts the mass he does work and transfers some energy to the mass which is stored* in the mass as potential energy. Once the bob has been lifted the weightlifter applies a force to keep the bob lifted (to counter the force exerted by the mass downwards i.e. mg) but does no work as the force does not cause any displacement. Edit: *No object stores potential energy; potential energy is the property of the system and not of a single object.

• No object stores potential energy; potential energy is the property of the system and not of a single object.
– user36790
Commented Jun 17, 2016 at 4:27
• @MAFIA36790 my bad, made the edit :) Commented Jun 24, 2016 at 7:21