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Atmospheric Pressure and Mercury in a Test Tube

  1. Aug 20, 2013 #1
    The problem statement


    Recently I came across this problem and couldn't understand it.

    I know that in situation 1, the length of the specific place will be decreased; In situation 2, it will be increased.

    Can someone help me here? Thanks first!
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Aug 20, 2013 #2


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    Staff: Mentor

    Welcome to the PF.

    With problems like this, you need to draw a free body diagram (FBD). For each situation, draw the forces that are acting on the mercury blob.
    Last edited by a moderator: May 6, 2017
  4. Aug 20, 2013 #3
    What holds the mercury in the specified locations in all the cases? Why the difference?
  5. Aug 20, 2013 #4
    Hi do you mean this?

    I need to know exactly what are the lengths for both specific situation...
    Last edited by a moderator: May 6, 2017
  6. Aug 20, 2013 #5
    Hi I'm not sure. I was given this problem in a practice. Maybe assuming there's an unknown gas inside? Situation 2 might be held by the atmospheric pressure? I have to find out the exact length covered by mercury.. Help!
  7. Aug 20, 2013 #6
    Observe that in situations 1, 2 and 3, in the first picture, atmospheric pressure is always present on the open end of the tube, and pushes the mercury inward. What is its force?

    But the mercury is stationary in all the cases. Why?
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