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Calculating to mass of a water column above a specific point

  1. Jan 16, 2016 #1
    upload_2016-1-16_16-36-9.png
    Part D. I looked up the answer and input it already, which is why it says correct, but i am beyond stumped. No idea how to do this. I know the pressure at the dotted lines are equal and that pressure=force/area. I also know the pressure at an area is equal to whatever is above it so the pressure on the left arm= 1atmosphere but im not sure if i would also include the 1kg weight. And I also know the pressure on the right arm is 1atm+ d*g*h and maybe + the kg weight. But I am so stumped on where to go from here
     

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  3. Jan 16, 2016 #2

    haruspex

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    Yes, you must include all the forces acting on the cap. Do that for both sides and you will get there.
    (How did you get the right answer for the first part?)
     
  4. Jan 16, 2016 #3
    i dont understand how to do this though. How do you add 1atmosphere to the 1kg weight?
     
  5. Jan 16, 2016 #4

    haruspex

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    1 atm is a pressure, 1kg is a mass. What force on the cap results from each?
     
  6. Jan 16, 2016 #5
    i honestly have no idea. Ive been working on this for at least an hour and have made no progress. If im supposed to incorporate all forces acting on this cap i dont get how to go that if im dealing with mass and pressure?
     
  7. Jan 16, 2016 #6

    haruspex

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    1. What is the relationship between pressure and force? You must have been given an equation connecting them.

    2. Why does placing a mass on top of something result in a force?
     
  8. Jan 16, 2016 #7
    i have Pressure=Force/Area. But that hasnt done me any good this far because I literally have no idea how to do this problem. If i plug in numbers than I have 1atm=1kg/1pi and I have nothing to solve for so I dont know how to use this equation in this context
     
  9. Jan 16, 2016 #8

    haruspex

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    It is a mistake to plug in numbers at the start. Always try to work symbolically. It makes the ideas clearer, and many other advantages.
    You are not told the actual areas, but you are told the ratio between them. The trick is to create a variable, A say, for the smaller area, so the large area is 4A.
    Let atmospheric pressure be Pa. Using your formula, what forces does the atmosphere exert on the two caps?

    Please try to answer question 2 in my previous post.
     
  10. Jan 16, 2016 #9
    ok, i give up, thanks though
     
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