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Homework Help: Absolute pressure

  1. Aug 23, 2011 #1
    1. The problem statement, all variables and given/known data
    the question is in the diagram i have attached. basically i have to find absolute pressure

    2. Relevant equations
    P = pgh where P is Pressure and p is density

    3. The attempt at a solution

    1st of all i need to convert 750mm into a pressure? somehow dont know exactly.

    the rule i was told is starting from the right side, if it goes down it is subtracted and if it goes up it is added. So in knowing that i did: p_atm - (8 * 0.15 ) - ( 10* 0.05) but i dont know the pressure yet coz i dont know how to calculate. is the rest of the method correct though?

    Attached Files:

  2. jcsd
  3. Aug 23, 2011 #2


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    Hi TyErd! :smile:
    (you mean 758 :wink:)

    758 mm of Hg means the pressure at the bottom of a tube of Hg 758mm high …

    so you need to know the density of Hg. :smile:
    Isn't it adding for both of them?

    You start with atmospheric https://www.physicsforums.com/library.php?do=view_item&itemid=80" and then it increases when you go down …

    don't you have to go down from the interface to that little arrow? :redface:
    Last edited by a moderator: Apr 26, 2017
  4. Aug 23, 2011 #3
    okay i get how to calculate the atmospheric pressure now. According to my book and a couple of questions completed with a teacher, If you are starting from the left, you add as you down and subtract as it goes up. And I think he said if you start from the right side, its add if you up and subtract if you go down. I'm not too sure, I need someone to confirm it thats right or not.
  5. Aug 24, 2011 #4


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    Hi TyErd! :smile:

    Forget left and right.

    You start with a place where you already know the pressure (usually because it's atmospheric pressure), then if you go down the pressure goes up (ie you add), and if you go up the pressure goes down (ie you subtract). :wink:
  6. Aug 24, 2011 #5
    thanks so much! makes a lot more sense now. you're the man
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