Suppose you are reshelving books in a library. As you lift a book from the floor and place it on the top shelf, two forces act on the book: your upward lifting force and the downward gravity force.

(a)Is the mechanical work done by your lifting force positive, zero, or negative?

positive zero negative

(b)Is the work done by the gravity force positive, zero, or negative?

positive zero negative

(c)Is the sum of the works done by both forces positive, zero, or negative?

positive zero negative