Hot air balloons rise because hot air inside the balloon
ABecomes denser and so floats on cool air below
BBecomes less dense and rises in the cool surrounding air
CHas the same density as the cool air outside
DHas more weight than the cool surrounding air
Answer & Solution
Correct answer: B. Becomes less dense and rises in the cool surrounding air
1. Heating expands a gas at constant pressure.
2. Same mass of air now fills a larger volume, lowering density.
3. Less dense hot air is buoyed up by denser surrounding air.
4. So the balloon rises.
_Source: Selina Concise Physics Class 8, Ch 5 Heat (Mistral OCR'd PDF), section on convection examples_
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