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Buoyant force on a submerged cube

  1. Jul 5, 2014 #1
    So a cube submerged in water will experience pressure on all six sides, and the pressure on the bottom will be greater than the pressure at the top (assuming there's gravity) and the cube will float to the top given that it has a low enough density.

    Something I'm curious about is what would occur if that cube is pushed down to the base of whatever vessel is containing the water. Now the pressure of the water cannot act on the bottom of the cube because the cube's bottom surface is in contact with the container, not the water. So would the cube just stay at the bottom and not float up?
     
  2. jcsd
  3. Jul 5, 2014 #2

    Dale

    Staff: Mentor

    If you actually remove some of the pressure on the bottom surface, then yes it will stay on the bottom and not float up. This is what happens for a drain plug or a suction cup, but typically just placing an object on the bottom doesn't actually reduce the pressure.
     
  4. Jul 5, 2014 #3
    I understand the principle of the suction cup and drain plug, but why won't pushing the cube to the bottom accomplish something similar assuming you are able to expel the water between the bottom of the cube and the container?
     
  5. Jul 5, 2014 #4

    A.T.

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    You have to expel and prevent it from flowing back in.
     
  6. Jul 5, 2014 #5
    What force would cause the water to flow back under? If the block and container are in contact, is there still some sort of low pressure area under the block?
     
  7. Jul 5, 2014 #6

    TumblingDice

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    Establishing this ideal might be harder than you think. It would require two perfectly polished surfaces to even consider attempting to setup a test. If you could create such a perfect seal that no water molecules could go below the cube. Then you could have something similar to the drain plug where pressure on topis greater than underneath.

    However, this sounds somewhere between extremely difficult, expensive, and/or impossible to easily demonstrate. :frown:

    EDIT: Removed comments regarding cube sides and pressure on sides.
     
    Last edited: Jul 6, 2014
  8. Jul 6, 2014 #7

    A.T.

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    Perfect contact is difficult to achieve, and the contact pressure would be lower than the water pressure from the sides (assuming a cube lighter than water).
     
  9. Jul 6, 2014 #8

    Delta²

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    Well one can glue the cube to the bottom of a tank, wait for the glue to dry and then fill the tank with water. Assuming that the glue doesnt alow flow of water within it, we would have achieve what we want. The role of the glue here is mainly to keep the water off the contact layer and not to neutralize any buoyant force. Since there would be no water beneath the cube the total pressure from the water will have a downward effect on the cube thus helping to keep the cube in place even more.
     
  10. Jul 7, 2014 #9

    Dale

    Staff: Mentor

    Think about the forces involved in expelling the water from between two rigid, smooth, and matching surfaces. How do those compare to the water pressure? What happens if they are lower?
     
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