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Question
A uniform metre rule rests horizontally on a knife-edge at the 60 cm mark when a mass of 10 gram is suspended from one end. At which end must this mass be suspended? What is the mass of the rule?
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Solution
Let M gram be the mass of the rule. Since the density of the rule is uniform, its weight Mg will act at its middle point i.e., at the 50 cm mark (Fig.).
The weight of the rule produces an anti-clockwise moment about the knife-edge O. In order to balance it, 10-gram mass is to be suspended at the end B, to produce a clockwise moment.
From the principle of moments,
Anti-clockwise moment = Clockwise moment
Mg × (60 – 50) = 10 g × (100-60)
or Mg × 10 = 10 g × 40
∴ M = 40 gram
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