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Another question (Buoyancy)

  1. Aug 3, 2004 #1
    Place a block at the bottom of a glass cup. The bottom of the block is polished smooth so that no air exists between the cup bottom and the block.

    Now fill the cup with water. Does the block rise?
     
  2. jcsd
  3. Aug 3, 2004 #2
    Try it, use a suction cup instead of a block. That's the effect you're looking for. I say it will not rise, even though it should float otherwise.
     
  4. Aug 4, 2004 #3
    need more detail^^

    Hi, Could you please explain with more detail?? Boyant force is always difficult concept for me. I understand the block has less density compare to water is assumed?? Then we would normally expect the block to float on the surface of the water if placed different way?? But now that the block was placed at the bottom of the cup and there's no empty gap b/t the cup and the bottom of the block. Even though there is some amount of water displaced by the block, no boyant force can be act on the block because there is no physical contact b/t the block's bottom surface and the water ??
    If my reasoning is wrong, correct me.
     
  5. Aug 4, 2004 #4

    russ_watters

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    Nope, you got it.
     
  6. Aug 4, 2004 #5
    Yeah, that's pretty much what I think.

    Well I don't think a real-life low-density "block" can actually stay underwater, the slightest nearly-molecular imperfection on the lower surface edge would allow water to flow underneath and push it upwards. You would need two perfectly smooth surfaces. I'm not sure whether it can or has been done.

    But if you stick a good, dry suction cup (rubber is less dense than water so usually floats) on the bottom of a dry glass, I suspect that you can slowly fill the glass up with water, so that the water pushes the suction cup against the bottom of the glass (like air did), preventing it to detach and float.
     
  7. Aug 4, 2004 #6

    Doc Al

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    You've certainly got the right idea. But if you define "buoyant force" as the force exerted by the water on the block, then in this special case the buoyant force is actually negative: the water pushes the block down.
     
  8. Aug 7, 2004 #7
    But there's a catch:

    When the block is at the bottom of the glass, the system has a relatively large potential energy compared to the situation where the block is floating.

    In fact, the potential energy of the system will decline once the block rises by ANY amount. Shouldn't there be a force associated with this motion since there is a change in potential energy associated with the motion?
     
  9. Aug 7, 2004 #8

    Doc Al

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    Isn't that just the "usual" buoyant force? (As soon as as the seal is broken, the block will have the usual upward buoyant force.)
     
  10. Aug 7, 2004 #9
    Absolutely. But does it take any force to break the seal? (There is no vacuum between the block and glass because there is no space between the block and glass.)
     
  11. Aug 8, 2004 #10

    Doc Al

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    Sure it takes a force to break the seal. You have to overcome the water pressure, right?
     
  12. Aug 8, 2004 #11
    Not if there is a buoyant force, and the change in potential that occurs when the block rises by any amount indicates that such a force exists.

    Or does it? As soon as the block rises, a vacuum should appear. At least in practice. But in theory, I'm not sure how it would work.
     
    Last edited: Aug 8, 2004
  13. Aug 9, 2004 #12

    pervect

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    I don't have any firm ideas as to how this experiment would turn out, but it seems to me that the surface effects will be very important.

    If the glass-water surface energy is lower than the glass-glass surface energy, I think that the water would "seep in" from the sides.

    Then there's the somewhat related issue of the Casimir force, a quick google seems to indicate that this force occurs not only between closely spaced conductors, but between closely spaced dielectrics as well.
     
  14. Aug 9, 2004 #13
    Surface energy?
     
  15. Aug 9, 2004 #14

    pervect

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  16. Aug 13, 2004 #15
    Let's assume that an otherwise bouyant cube of whatever material in glass of water was placed in the bottom of the container, such that there was a significant surface contact with that cube and the bottom of the glass.
    Fine.
    It should be considered that as the "cube" is originally bouyant within the environment, a certain amount of force is required to place it in the condition you describe(on the bottom of the filled container)
    The force required to change that position to normal bouyancy MUST exceed the force required to place it in it's static position on the bottom. If the object is "locked" by vacuum or molecular attraction with respect to contact with the bottom container surface, it does not matter. What matters are the forces potentially acting upon it to dislodge the condition, and if they are strong enough to do it.
     
  17. Aug 15, 2004 #16
    Why? Doesn't a buoyant force exist because the vector sum of the forces caused by the greater pressure water on the bottom of the object exceeds in magnitude of the vector sum of the forces from the lower pressure water on top of it? Why would you have to block off all of the higher-pressure forces on the bottom?
     
    Last edited: Aug 15, 2004
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