A 1.60-m-tall person lifts a 1.65-kg book off the ground so it is 2.20 m above the ground. What is the potential energy of the book relative to

VIDEO TRANSCRIPT

This is Giancoli Answers with Mr. Dychko. This person lifts a book, 2.2 meters up into the air; the person's 1.6 meters tall and the separation between the person's head and the book is 2.2 minus 1.6 meters, which is 0.6 meters and we'll need to use this value in part (b) of the question. Part (a) of the question is, "what is the potential energy, relative to the ground?" So that's gonna be mg times height above the ground; the ground is the reference level for part (a). So we have 1.65 kilograms— mass of the book— times 9.8 newtons per kilogram times 2.20 meters which is 35.6 joules of potential energy relative to the ground. Potential energy, relative to the head, is the mg times the height above the head; the head is the reference level now so that's the mass times g times 0.60 meters which is 9.70 joules. And the work done by the person equals the change in potential energy that occurred over the distance during which the force of the person was applied, and the person applied the force beginning at the ground and up to this final height here so it's that change in potential energy that matters and so it has only to do with part (a). Part (b) is not relevant for, as for answering the question of how much work the person did because, yeah, you need to consider the total height over which the person applied the force, which is starting from the ground and going up to this, 2.2 meters above the ground. And so the answer is the same as the answer in part (a); the work done by the person is what's causing this change in potential energy and they do 35.6 joules of work.