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## Main Question or Discussion Point

In my text, we learn that the work done on an object is equal to the change in kinetic energy of that object. It seems to me that this statement requires some restrictions. If I take a an object at rest of mass m and lift to a height h and place it at rest on a shelf, the work I've done is -mgh. But I haven't changed the kinetic energy.

If you consider some U-K system (sorry), perhaps where U is given by mgh, then the work done by the conservative force which provides U is always equal to the negative of the change in kinetic energy, provided there are no other forces.

In the first example, the total energy changes and W=deltaK fails. In the second, the total energy is constant and W=deltaK holds. How do I know in general when to apply the work-energy theorem??

If you consider some U-K system (sorry), perhaps where U is given by mgh, then the work done by the conservative force which provides U is always equal to the negative of the change in kinetic energy, provided there are no other forces.

In the first example, the total energy changes and W=deltaK fails. In the second, the total energy is constant and W=deltaK holds. How do I know in general when to apply the work-energy theorem??