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When does the plate of glass loosen from a glass cylinder in water?

  1. Sep 25, 2014 #1
    1. The problem statement, all variables and given/known data
    A thin glass plate is pressed at the end of a glass cylinder. The glass cylinder with the glass plate at the bottom is dipped in water so that the plate is 25 cm under the water surface (The glass cylinder i open, meaning only the bottom part is closed, due to the glass plate). They we proceed with filling this cylinder with Ethanol, and the question is: how much Ethanol must we fill until the plate loosens. It asks for the height of Ethanol, meaning how high must we fill it.

    (I realize that this is not the easiest way to describe the problem, but being that my English is not that good, this is the best I can do. Sorry in advance)

    Density of Ethanol: 0.79 * 10^3 kg/m^3
    of Water: 1.00 * 10^3 kg/m^3

    3. The attempt at a solution
    I don't really know, I looked at the proposed solution, but I understand nothing. The book never said anything else about this. The proposed solution said that the plates loosens when the pressure of water = pressure of ethanol, basically: D(water) * g * h (25 cm) = D(ethanol) * g * h (this is what we are supposed to find).

    I don't understand why...
     
  2. jcsd
  3. Sep 25, 2014 #2

    Simon Bridge

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    Ignoring the model solution: just thinking about it for a bit - what are the forces on the glass plate?
     
  4. Sep 26, 2014 #3
    It says nothing about what kinds of forces that are in play. The only thing we get to know is that: the plate of glass is pushed at the end of the cylinder of glass which is then put under water.
     
  5. Sep 26, 2014 #4

    ehild

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    Think of hydrostatic pressure ....

    ehild
     
  6. Sep 26, 2014 #5

    Simon Bridge

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    ... if the glass end was not under water, it would just fall off.
    Why? What is it about being under water that holds the glass plate in place?
    (As ehild points out: think - hydrostatic pressure... you'll see you are told a lot about the forces on the plate.)

    This question does not come just out of the blue: you have been doing some coursework as well. What is the subject you are learning in class about now?
     
  7. Sep 29, 2014 #6
    I understood it now. I mistook some information given by the book. I wasn't able to see the difference on p and greek letter ro. I understand now that there is pressure or force affecting the whole glass cylinder, both from the water on the glass cylinder and the ethanol on the glass cylinder.
     
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