Question

A uniform metre rule AB is balanced horizontally on a knife edge placed 15cm from A with a mass of 30g at A. What is the:
A. Mass of the rule.
B. The force exerted on the rule by the knife edge.

Answer 1

M = 450/10 = 45

Explanation:

For moment of mass, the stick’s mass M gram can be considered concentrated at its center of mass, its middle 25 cm from A. Its moment about the knife edge at 15 cm from A is:

M(25–15) g.cm …………. [1]

The opposing 30 gram mass is located 0 cm from A. Its moment about the knife edge at 15 cm from A is:

30(0–15) g.cm ………. [2]

The sum of moments, equation 1 + equation 2, must equal 0 if they balance on the knife edge:

M(25–15) + 30(0–15) = 0

M*10 -450 = 0

M = 450/10 = 45 gram is half-meter stick’s mass. I do not know about the whole meter stick.