Consider the two cases:
a. A says that the potential energy of a book placed on a shelf is `10J`.
b. B says that the potential energy of the same book is `-10J`.
Which one of them is wrong? Justify.

Explanation

To determine which statement is correct regarding the potential energy of a book placed on a shelf, we need to analyze the situation based on the concept of potential energy and the frame of reference.

### Step-by-Step Solution:

1. **Understanding Potential Energy**:
Potential energy (PE) is defined as the energy an object possesses due to its position in a gravitational field. The formula for gravitational potential energy is given by:
\[
PE = mgh
...