(solved)Question 2.12 of NCERT Class XI Physics Chapter 2

The mass of a box measured by a grocer's balance is 2.30 kg. Two gold pieces of masses 20.15 g and 20.17 g are added to the box. What is (a) the total mass of the box, (b) the difference in the masses of the pieces to correct significant figures.

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Parveen,

Question 2.12
NCERT Class XI Physics

The mass of a box measured by a grocer's balance is 2.30 kg. Two gold pieces of masses 20.15 g and 20.17 g are added to the box. What is (a) the total mass of the box, (b) the difference in the masses of the pieces to correct significant figures

Video Explanation
(detailed solution given after this video)

Please watch this youtube video for a quick explanation of the solution:

Solution

During addition and subtraction decimal places have to be taken into account - NOT the significant figures!

$\displaystyle $

$ \begin{aligned} &\text{mass of box } m_b = 2.30 \equiv 2.3 \text{ kg} \\ &\text{it has } \textcolor{red}{\underline{1}} \text{ decimal digits}\\\\ &\text{mass } m_1 = 20.15 \text{ g} \equiv 0.02015 \text{ kg} \\ &\text{it has } \textcolor{red}{\underline{5}} \text{ decimal digits}\\\\ &\text{mass } m_2 = 20.17 \text{ g} \equiv 0.02017 \text{ kg} \\ &\text{it has } \textcolor{red}{\underline{5}} \text{ decimal digits}\\\\ &\text{add }2.3 + 0.02015 + 0.02017 = 2.3402\\ &\text{decimal digits to be min(1, 5, 5) = 1}\\\\ &\text{hence round to 1 place} = 2.3 \text{ kg}\:\underline{Ans}\\\\ &\text{(b) difference } 20.17 - 20.15 = 0.02\\\\ &\text{decimal places to be 2}\\\\ &\therefore 0.02 \text{ gm}\:\underline{Ans} \end{aligned} $

Confusion Explained

If you are confused about the answer in the book and the answer that you and your friends are getting, then watch this video:

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