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The area of cross section of the wider tube shown in figure is \( 900 \mathrm{cm}^{2} \). If the boy standing on the piston weighs 45 kg, the difference in the levels of water in the two tubes.

1. \( 50 \mathrm{cm} \)
2. 25cm
3. \( 75 \mathrm{cm} \)
4. 100 \( \mathrm{cm} \)

Correct answer: \( 75 \mathrm{cm} \)

Solution

The boy adds pressure on the wider piston equal to his weight divided by the piston area. In a connected fluid, that extra pressure is balanced by the pressure from a vertical water column, so the level difference comes from equating these two pressures.

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