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A mercury barometer inside an elevator that is accelerating upward gives a reading of 76 cm. What is the actual air pressure inside the elevator?

1. 76 cm
2. less than 76 cm
3. greater than 76 cm
4. zero

Correct answer: greater than 76 cm

Solution

When the lift accelerates upward, the effective acceleration on the mercury is (g + a) > g. The air pressure must support a column whose pressure is rho*(g+a)*h. A reading of h = 76 cm under enhanced gravity corresponds to a pressure rho*(g+a)*76, which exceeds the standard pressure of 76 cm of mercury (rho*g*76). So the true air pressure is greater than 76 cm.

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