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Homework Help: Fluids Help!

  1. May 7, 2008 #1
    1. The problem statement, all variables and given/known data

    A Partially evacuated airtight container has a tight-fitting lid of surface area 77m^2 and negligible mass. If the force required to remove the lid is 480N and the atmospheric pressure is 1x10^5 Pa, what is the air pressure inside the container?

    2. Relevant equations


    3. The attempt at a solution



    This did not make sense to me, so i thought maybe it was the difference in pressures:


    I don't think that is right either. Where I am going wrong?
  2. jcsd
  3. May 7, 2008 #2
    It always helps to start off by balancing forces. There is a downward force on the lid due to the motion of the particles in the atmosphere (F_A). There is an upward force on the lid due to the motions of the particles in the container (F_C). And, finally there is a normal force exerted by the walls of the container on the lid - also in the upward direction (F_N). Maybe you can imagine this more easily by covering the container with a flat metal sheet instead of with a screw-top lid.

    In the static case where you aren't doing anything to the container the forces are balanced:
    F_C + F_N - F_A = 0

    In order to remove the lid you need to apply an additional upward force (F_you)

    If F_you is too small, you will only decrease the size of the normal force that the container walls have to exert to support the atmosphere pressing down on the lid: F_N (F_N goes to F'_N):

    F_C + F_you + F'_N - F_A = 0

    What happens when F_you is big enough when F'_N goes to 0?
  4. May 7, 2008 #3
    Would the lid be removed at that point because F_you cancels out F_N?
  5. May 7, 2008 #4
    I'm lost on this. Any other suggestions?
  6. May 7, 2008 #5


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    Staff Emeritus
    Science Advisor
    Homework Helper

    I think jinman's method is correct, but the input numbers as given are wrong somewhere.

    77m^2 is an awfuly large size for a container lid. Reread the question in the book (or wherever you got it from) to see if that figure should really be something else.
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