physics- please explain
posted by Anya on .
Why is the following situation impossible? A librarian lifts a book from the ground to a high shelf, doing 20.0 J of work in the lifting process. As he turns his back, the book falls off the shelf back to the ground. The gravitational force from the Earth on the book does 20.0 J of work on the book while it falls. Because the work done was 20.0 J + 20.0 J = 40.0 J, the book hits the ground with 40.0 J of kinetic energy.
i don't understand why it is impossible. :/ any help appreciated!
The work done by gravity would be -20.0 J in relation to the work done by Librarian.
*The Librarian lifts the book UPWARD doing 20.0 J of work.
*Gravity pulls the book DOWARD at -20.0 J of work RELATIVE to that of the Librarian's work.
*The book hits the ground with 20.0 J of kinetic energy. WHY? -- Gravity pulled the book DOWNWARD with 20.0 J of work.
(It would be -20.0 J if the question asked for the kinetic energy to be relative to the Librarian's work done).