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Buoyant Force at the bottom of a pool?

  1. May 14, 2014 #1

    Nathanael

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    Apparently, Buoyant Force is caused by a difference in water pressure.

    So am I correct in thinking that this means if you have a cube (chosen so that buoyant force does not come from the sides, just from the bottom) at the bottom of a pool, there would be no buoyant force?

    How could there be a buoyant force if there's no water below it pushing up?

    So if the bottom of the pool were a scale, and the cube were all the way on the bottom (with no water between) doesn't that mean it would actually weigh more? (normal weight + weight from water pressure above)
    And that when people ask that hypothetical question, "What would you weigh on a scale underwater?" and expect the answer to be your weight minus the buoyant force, wouldn't that answer only be approximately true? (Since some of the buoyant force from below your feet doesn't exist. It would still be approximate though because the majority of the buoyant force would likely come from the irregular shape of the human body.)

    I have a feeling that I'm wrong in thinking this but I cannot seem to figure out how there would be a buoyant force (on a cube) without water being beneath it.
     
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  3. May 14, 2014 #2

    boneh3ad

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    You aren't wrong provided that you make certain there is no fluid between the flat face of the object and the bottom surface.
     
  4. May 14, 2014 #3

    Nugatory

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    Of course it's hard to keep the fluid from getting between the flat face and the bottom of the pool, as there's a fair amount of water pressure pushing it in there. But it can be done... Think about a suction cup stuck to the bottom of the pool.
     
  5. May 14, 2014 #4

    jbriggs444

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    Your apparent weight as measured by such a scale would be given by your true weight minus the weight of the fluid that you displace regardless of whether you are wearing golf shoes with sharpened spikes, shoes with flippers or shoes with suction cups.

    If you don't want to equate the weight of the fluid that is displaced with the "buoyant force", that's fine. But that leaves you with no easy way to compute the "buoyant force" and no obvious reason to need to do so. So I see no good reason to adopt such terminology.
     
  6. May 14, 2014 #5

    boneh3ad

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    The problem is that this works well for a completely immersed body, not so well if the a portion of the body is "dry" due to being up against the bottom. Sure in that situation it is still displacing water, but for an upward force to occur, to which surface does that upward force apply? The buoyant force is ultimately a fluid force and as such can only occur on faces that are exposed to the fluid. Buoyancy ultimately arises from the hydrostatic pressure, so the only way to get an upward buoyant force is for there to be some surface with a nonzero normal component in the downward direction that is exposed to the fluid. A cube with a perfectly dry bottom face does not have such a surface.
     
  7. May 14, 2014 #6

    jbriggs444

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    Makes sense, thanks. Nugatory's suction cup would be an example where computing the force required to lift an object by naively taking gross weight minus buoyancy would be woefully inadequate.
     
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