Question

Adita lifts a book from the floor, carries it across the room, and places it on a high shelf. When is Adita doing work on the book?

She does work on the book at no time during this process.
She does work only while she carries the book across the room.
She does work from the moment she touches the book until she lets it go.
She does work as she picks up the book and as she lifts it to the shelf.

Answer 1

She does work as she picks up the book and as she lifts it to the shelf.

Explanation: Mechanical work is force times displacement in the direction of the force. When she lifts the book upward she applies an upward force while the book moves upward, so she does positive work (increasing gravitational potential energy). While she carries it horizontally at constant height, her upward force is perpendicular to the displacement, so she does no work. Holding it without moving also does no work.