physics- TEST TMRW PLS HELP!

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!

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  1. 20 J of gravitational potential energy is lost as the book falls. Its kinetic energy (KE) as it hits the ground is 20 J.

    It has zero KE on the high shelf and regains 20 J on the way back down.

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  2. thank you so much!

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