A metre stick is balanced on a knife edge at its centre. When four coins, each of mass $2 \mathrm{~g}$ are put one on top of the other at $10.0 \mathrm{~cm}$ mark, the stick it found to be balanced at $46.0 \mathrm{~cm}$ mark. The mass of the metre stick is

• A

  66 g
• B

  60 g
• Correct answer
  C

  72 g
• D

  18 g

Solution

$\begin{aligned} & \text { As } \mathrm{J}_{\mathrm{CW}}=\mathrm{J}_{\mathrm{ACW}} \\ & \Rightarrow \quad \mathrm{mg} \times 4=8 \times 10^{-3} \mathrm{~g} \times 36 \\ & \Rightarrow \quad 4 \mathrm{~m}=8 \times 10^{-3} \times 36 \\ & \Rightarrow \quad \mathrm{m}=72 \times 10^{-3} \mathrm{~kg}=72 \mathrm{gm}\end{aligned}$

Hence, the correct answer is option: 'C'.