Equal masses of 100 kg of iron and of cotton are weighed separately with a spring balance at the Earth's surface. If R1 is the reading for iron and R2 the reading for cotton, which relation holds?

R1 < R2
R1 = R2
R1 > R2
R1 = R2 = 0

Correct answer: R1 > R2

Solution

The spring balance reads apparent weight = mg - buoyant force of air. Cotton has much lower density, hence much larger volume, so it displaces more air and experiences greater upthrust. Its reading R2 is therefore smaller than R1 of iron, giving R1 > R2.

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