Question

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

Answer 1

Figure shows all the forces on the stick

The mass of metre stick is concentrated at CM = 50cm Consider the torques around 0. 

Since the stick is in rotational equilibrium,equilibrium,

\(\bar τ_{ext}\)= m (5cm) – (10g) (45 – 12) = 0

∴ m = 10g \(\frac{(33cm)}{5cm}\) = 66 gm.