Question

A book is lifted from the floor and is kept in an almirah. One person says that the potential energy of the book is increased by 20 J and the other says it is increased by 30 J. Is one of them necessarily wrong?

Answer 1

Answer ⇒ Yes. 

Explanation ⇒ 
As per as the question, a book is lifted from the floor and is kept in the Almirah. Now, It means that the book is kept at some height. This means it will have the constant Potential energy.  
Assume mass of the book as x grams. 
∴ Potential Energy of the Book in the Almirah = x × 10 × h.

Now, If one person is saying that the Potential Energy is 30 J.
This means that, 
mass of the body = 30/10h
 = 3/h. 
Another Person is saying that the Potential energy is 20 J.
This means that,
mass of the body = 20/10h
= 2/h

Mass in both the case is different. It cannot be Possible, since mass will never changes. 

Hence, one of the Person is necessarily wrong.


Hope it helps.

Answer 2

HEY!!

______________________________

⏩Yes, one of them is necessarily wrong.

⏩We measure potential energy from a reference level chosen by the observer.

⏩However, the change in potential energy of a body does not depend on the level of reference.