Question

A 1.60m tall person lifts a 2.10 kg book from the ground so it is 2.20 m above the ground. What is the potential energy of the book, relative to: (a) the ground and (b) the top of the person's head? (c) How is the work done by the person related to the answer sin parts (a) and (b)?

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asked 2020-10-18
A 1.60m tall person lifts a 2.10 kg book from the ground so it is 2.20 m above the ground. What is the potential energy of the book, relative to:
(a) the ground and
(b) the top of the person's head?
(c) How is the work done by the person related to the answer sin parts (a) and (b)?

Answers (1)

2020-10-19
Potential energy is given by mgh, where h is height. The problem is, height is relative and when we deal with PE we have to define where height is zero. So parts a and b address this issue. For part a, the ground level is zero and the book is 2.20 m abovethe ground. Calc PE using 2.20 for height.
For part b, the idea is that now the persons head is defined as the zero level, so the book is only at a height of 0.60 m. Use this value to calc PE.
Part c... the amount of work the person does is independent o four definition of ground level. No matter where we define h to be zero, the person still lifts the book a distance of 2.20 meters, so this is the distance used to calc the amount of work done. Incidentally, work is the change in PE when lifting an object. If we define the ground level as zero, then initially the book has zero PE. Then the answer to part a, the PE relative to the ground,does happen to equal the amount of work done.
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