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Does Mass Occupying Volume in a Compressed Air System Transfer Energy

  1. May 2, 2013 #1
    In a compressed air space there is a suspended mass with 10 cubic meters volume. The pressure vessel is 20 atm. The suspended mass is suspended 100 meters over water that is kept at bay by the air pressure inside the vessel.

    The moment the mass is released its potential energy converts to kinetic energy.

    There is a door that closes off the 10 cubic meters of volume space so that there is 20 atm of pressure inside (E=PV).

    The mass falls and impacts the water and submerges.
    The water level rises.

    Does the pressure inside the vessel decrease by 10m3 x 20 atm?

    Is the PV energy inside the closed space equal to 10m3 x 20 atm minus air resistance?

    Can this be written in math?
    Can this idea be expressed mathematically?
  2. jcsd
  3. May 2, 2013 #2


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    The water will not be kept "at bay" by the air if there is no gravity. Think of what happens to water in the Space Shuttle or Space Station in orbit...

    And if this experiment is carried out on the surface of the Earth, the volume of the air stays the same before and after the mass goes underwater. The surface of the water only rises because of the volume of the mass that is submerged, so the total volume of the water + object is constant.
  4. May 3, 2013 #3


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    What berkeman said. The volume of air stays the same. You just traded the volume occupied by the mass (10cubic meters) for the volume occupied by the water (10 cubic meters).

    The mass had some PE which was converted to KE and then to heat when it hit the water so I suppose the water warmed slightly.
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