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Homework Help: Liquid pressure in microgravity

  1. Aug 1, 2013 #1
    1. The problem statement, all variables and given/known data

    This is more of a conceptual question rather than a homework problem. This is my first post, so apologies if this is in the wrong section.

    Consider a jar full of liquid in microgravity.

    My book says:
    According to the formula p = ρgh, p→0 as g→0. Thus there is no pressure in the jar when it's in microgravity.

    My problem:
    Isn't it true, though, that the molecules in the liquid are jiggling around? Wouldn't these molecules still inadvertently bump into the sides of the jar, creating pressure?

    2. Relevant equations

    p = ρgh

    3. The attempt at a solution

    My current thought is that the pressure due to this bumping is negligible. So the formula p = ρgh is a (good) approximation.
  2. jcsd
  3. Aug 1, 2013 #2
    Here is my opinion. "the formula p = ρgh" is generally used in determining the pressure of liquids at a given depth, h, in a container on the Earth.
    I would think that in no gravity, like in space, there will always be some pressure due to the vapor of the liquid. If you had a gas in space, then of course there would be pressure.
  4. Aug 1, 2013 #3
    Barryj is correct. In addition, the equation p=ρgh is not really correct. It should not be used to determine the absolute pressure. The equation should really read Δp=ρgΔz, or, even better dp = ρg dz. The equation gives the relative change in pressure with distance in the direction of the gravitational vector, rather than giving the absolute pressure.
  5. Aug 1, 2013 #4
    Brilliant, thank you barryj and Chestermiller.

    Would it be appropriate to suggest [itex] p = p_{0} + \rho gh [/itex], where [itex] p_{0} [/itex] is the initial pressure? So, as you were saying Chestermiller, [itex] p = \rho gh [/itex] would be the change in pressure.
  6. Aug 2, 2013 #5
    Yes, with spatial position in a gravitational field.
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