If Work = Force x Displacement, how come you still have to do work (use energy) if you want to keep a weight at a fixed height?

Answer

Dear Wilbert,

You have to distinguish between the concept of “work” as it is used in physics and what we mean by “work” in the conventional language.

The (physical) quantity “work” is a very precisely defined concept from mechanics and, as you write, it is equal to the force times displacement. If you hold an object at a fixed height, no work is done on the object.

But you’re right that our muscles still have to expend energy to keep the object at the same height. However, that is not “work” in the physical sense of the word.

Kind regards,

Philippe Tassin