In my text book there is given an example of a person holding a chair for three minutes. The persons arm gets tired but no work is done because there is no displacement. In the sidebar of my textbook, there is this note: "We can calculate the work done by a force on an object, but that force is not necessarily the cause of the obect's displacement. For example, if you lift an object, (negative) work is done on the object by the gravitational force, although gravity is not the cause of the object moving upward!" So if in the case of the person holding up a chair, if gravity is doing negative work, then for there to be net zero work done, there must be some positive work done by the person. So how can they say there is no work done here?