Rekha has a metre ruler weighing 200 gf, half (50 cm) made of wood and half made of steel.
(a) Rekha wants to balance the ruler. To which part (wooden or steel) should she keep the knife edge?
(b) The above metre-rule is to be balanced by placing the knife edge at 50 cm mark using a known weight. Name the principle which can be used to calculate the position of the weight.
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(a) The knife edge should be kept on the steel part.
Reason — Steel is much denser than wood, so for the same length the steel half is heavier than the wooden half. Hence the centre of gravity of the ruler is shifted from the 50 cm mark towards the steel half, and the ruler will balance only when the knife edge is placed at the centre of gravity, i.e., somewhere on the steel part.
(b) Principle of moments — according to which, in the equilibrium position, the sum of the anticlockwise moments about the knife edge is equal to the sum of the clockwise moments about it.