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Fluid Mechanics — Physics STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 SciencePhysicsFluid Mechanics1 Mark
Question
A barometer kept in an elevator accelerating upward reads 76cm. The air pressure in the elevator is
  1. 76cm.
  2. < 76cm.
  3. > 76cm.
  4. zero.
✓

Answer

  1. > 76 cm
Explanation:
When the elevator is going upwards with acceleration a, the effective acceleration is a' = (g + a).
Thus, pressure is given by:
$\text{P}=\text{h}\rho(\text{g}+\text{a})$
Air pressure in the elevator $=\text{p}=\text{h}'\rho\text{g}$
Because the pressure is the same, h' > h.
$\therefore$ Air pressure > 76cm.

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