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Will the barometer reading rise or fall ?

  1. Feb 28, 2006 #1
    If i have a barometer which reads 76cm of hg when kept in an elevator when the elevator is at rest . now the elevator starts going up with some acceleration . will the barometer read less than , greater than or equal to 76cm of hg . also explain why . what will happen if the barometer is under free fall ?
     
  2. jcsd
  3. Feb 28, 2006 #2

    Astronuc

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    Staff: Mentor

    How does a barometer work?

    What is significant about the height of a column of liquid and the pressure in the liquid?
     
  4. Feb 28, 2006 #3
    If the elevator is going in the upward direction with some acceleration the liquid level will rise ? am i right ? but what will happen if the elevator is under free fall ? will the reading become infinity or zero ? what does that phyically signify ?
     
  5. Mar 1, 2006 #4

    Astronuc

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    Staff: Mentor

    The column of liquid in the barometer imposes a downward force (weight) = mg, and the pressure at the base of the column is rho * h * g (or rho * g * h). This pressure is balanced by the atmospheric pressure pushing down on a pool of liquid at the base of the column or end of the liquid column if there is a 'U' bend in the barometer (the sealed end (above column of liquid) of the barometer should have a vacuum).

    When the elevator 'accelerates' upward, the column of liquid experiences an effective weight of m(g+a), producing a pressure at the base of the column of rho * (g+a) * h. When the elevator 'accelerates' downward, the weight is reduced, m (g-a).

    In free fall, the elevator is accelerating at the rate of g, so the column has no weight and the atmospheric pressure pushes the column up into the barometer and there is some pressure on the container. But it is not infinite - it is limited by the pressure of the atmosphere pushing on the pool of liquid at the base of the barometer.
     
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