A uniform meter rule is balanced horizontally on a knife edge placed at the 40 cm mark when a mass of 60 g is suspended from the 10 cm mark. What is the total mass of the meter rule?
Answer and explanation
Correct answer: A. 180 g
- Step 1: The center of gravity of a uniform meter rule lies at 50 cm. Step 2. Distance from knife edge (40 cm) to suspended mass (10 cm) is 40 - 10 = 30 cm. Distance from knife edge to center of gravity (50 cm) is 50 - 40 = 10 cm. Step 3. Principle of moments about pivot: 60 × 30 = M × 10 => 1800 = 10M => M = 180 g. Answer: 180 g. Why not the others: 120 g uses 20 cm distance; 240 g uses 40 cm distance; 90 g halves the required mass.
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