Question

A metre stick is balanced on a knife edge at its centre. When two coins, each of mass $5g$ are put one on top of the other at the $12.0cm$ mark, the stick is found to be balanced at $45.0cm$. What is the mass of the metre stick in gram?

Answer 1

Given: $m_{coin}=5g$

Let $m$ be mass of the metre stick

As the metre stick balances at $50cm$ mark, its $CG$ must lie at this mark.

The $45cm$ mark is the $CG$ of the metre stick $+2$ coins system

Distance between $50cm$ mark and new $CG=50–45=5cm$

Distance between $12cm$ mark and new $CG=45–12=33cm$

From the principle of moments (for equilibrium),

$mg\times 5=2m_{coin}g\times 33$

$m=66grams$

FINAL ANSWER: $66$