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Fluid and Forces

  1. Sep 4, 2014 #1
    1. The problem statement, all variables and given/known data

    There is a figure of a cylinder of liquid partially immersed in a beaker of a fluid.

    Why is there a force that pushes up on the cylinder of liquid in the beaker?

    Multiple choice:
    1. The weight of the liquid cylinder and the atmospheric pressure acting on the cylinder is balanced by a force the rest of the liquid exerts back on the cylinder.

    2. Normal force the glass exerts back on the liquid.

    3. The weight of the liquid cylinder is balanced by a force the rest of the liquid exerts back on the cylinder.

    4. Atmospheric pressure.

    2. Relevant equations

    None

    3. The attempt at a solution

    I think it is 3 because atmospheric pressure doesn't change I think because the surface area for the air to push down stays the same which would eliminate 1 and 4. 2 Doesn't seem like that is what happens.

    I know the forces acting on the cylinder are:

    Down: Weight of cylinder + atmospheric pressure
    Up: Fluid on cylinder (I think it is buoyancy)

    I really need help, been stuck thinking about this for a long time. My teacher also gave me a hint that is wasn't #1.
     
  2. jcsd
  3. Sep 5, 2014 #2

    andrevdh

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    What does Archimede's principle say?
     
  4. Sep 5, 2014 #3
    "Archimedes’ principle, physical law of buoyancy, stating that any body completely or partially submerged in a fluid (gas or liquid) at rest is acted upon by an upward, or buoyant, force the magnitude of which is equal to the weight of the fluid displaced by the body."

    I actually haven't even heard about this yet! So it would be #3 because the weight of the cylinder of liquid displaces the water and so the water creates the upward buoyant force. Thanks!
     
  5. Sep 5, 2014 #4

    haruspex

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    Have you quoted the question exactly, word for word?
    The first part discriminates between liquid in the cylinder and 'fluid' outside it. There is no further mention of 'fluid'. If the distinction matters then all answers are wrong. If it doesn't matter then it's hard to know what some of the answers intend.
    The question asks why a certain force exists. All of the answers relate to forces balancing. The problem statement does not say anything is in balance.
     
  6. Sep 8, 2014 #5

    andrevdh

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    That's it, yes. The surrounding water previously supported the water before it was displaced and do now support the cylinder and its contents. This is the buoyant force. So a body will sink until the water it displaces is equal to its own weight, unless its weight is more than the weight of water with a volume equal to the object's total volume.
     
  7. Sep 8, 2014 #6

    haruspex

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    That's all true, but the question and offered answers still don't make much sense.
    If I reword it thus:
    A cylinder containing a liquid, A, floats in a second liquid, B. What force enables it to float?​
    ... And reword the first answer as
    The weight of the cylinder plus liquid A, plus the atmospheric pressure acting on the cylinder, is balanced by the forces on the cylinder from liquid B.​
    Then that would be the answer.
     
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