Question

A uniform metre rule of mass 150 g is placed on a knife edge at 70 cm mark. A body of mass 50 g is hung from
20 cm mark. Another body of mass 300 g is hung from 90 cm mark.
(a) Find out the
() total clock wise moment about the knife edge
(ii) total anti-clockwise moment about the knife edge
(iii) total moment of force about the knife edge
b) Where should a body of mass 250 g be placed on the scale to balance the scale?​

Answer 1

Explanation:

Let M gram be the mass of the rule. Since 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