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

In fact, both A and B could be right. For calculating potential energy of a body, a reference frame is chosen. Remember that potential energy is relative. You can calculate potential energy of a body with respect to a reference. In this case, the reference frames happen to be above and below the shelf. A calculated the potential energy of the book with respect to ground while B calculated the potential energy with respect to the ceiling. Hence, when potential energy is calculated, it is important to specify the reference frame with respect to which it is calculated.