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What is buoyancy?

  1. Apr 23, 2010 #1
    Hello everyone,

    I'm bit confused about buoyancy. First of all I want to know if it is newton's third law, filling a vaccum or none of above. For example these are my 3 diffferent understandings of what buoyancy could be. Which one is right

    I fall on water and diplace water molecules, do the water molecules give a reaction force back on me calling buoyancy. If that is the case why is it not Newton's third law.

    when I displace water molecules, water molecules from underneath come to fill the vaccum and push me up(especially when you displace air isn't this what happens). Is this buoyacny?

    Thanks :smile:
  2. jcsd
  3. Apr 23, 2010 #2
    When an "object" has a greater density than the fluid it will sink until that density is equal to the fluid at that level.
    Does that help?

    Conversely, an object will float or rise if its density is less than the fluid surrounding it.
  4. Apr 24, 2010 #3
    It's best to think of it like this. If you pour mercury into water, the mercury flows to the bottom of the container because it's denser than water. Same thing for a lead brick.

    Now if you pour water onto mercury it stays on top. Same for an ice cube. It's not dense enough to push the mercury aside and sink. Pit differently gravity pulls on the mercury harder than on an ice cube so the ice cube stays on top. And if you push the ice cube to the bottom of the mercury bucket and let it go, the cube will float back up to the top because the heavier mercury is pulled down and under the ice cube with greater force by gravity.

    That's the heuristic explanation. The correct way to describe it is that in a fluid there's a pressure gradient from the bottom to the top. You feel this gradient in tour chest if you dive into a swimming pool. So the pressure under a submerged object is higher than the pressure above it. If the submerged object weighs less than this pressure difference, the object is pushed up. If the object is greater than this difference it will sink.
  5. Apr 24, 2010 #4
    Thanks antiphon :smile: Ok so if I get a water bottle, are the molecules at the bottom travelling towards the top because of the pressure difference? So then if I submerge an object with large volume, it will displace water to a greater depth, hence it has to overcome a higher pressure difference to sink. Is that what happens.
  6. Apr 24, 2010 #5
    In your water bottle the water isn't moving. This is the study of hydrostatics. There is a steady pressure gradient but nothing moves becuase the pressures all balence the weights.

    Check out the hydrostatics page in Wikipedia.
  7. Apr 24, 2010 #6
    Then if nothing moves up, how is the buoyancy force created upwards? I looked at the hydrostatic page but unfortuanetly I don't understand much about it. Can you tell me why nothing moves, because normally pressure gradient creates movement. Thanks :smile:

    EDIT: In buyoncy are the displaced molecules following the pressure gradient to create the upward force. So when the molecules are dispersed are they coming back to push the object.
  8. Apr 24, 2010 #7
    Why does hot air rise?

    Hello everyone,

    I know the pressure at bottom of atmosphere is higher than above. Then when I suddenly heat up air at the bottom of atmosphere why does it rise? Ok if it is buoyancy, why doesn't the air at the top fall down towards earth, because they are less dense so they must fall right? I can understand how a small leaf can float in air but how can the medium itself float up? Thanks :smile:
  9. Apr 24, 2010 #8
    For example water consists of molecules which all whizz around and collide with the object submerged in the water. These collisions give the object a push, but since they come from all directions, the object doesn't move on average.
    Well... not quite. As you know the density of molecules increases with water depth. So more molecules are colliding against the object from below than from above. The net effect is an acceleration upwards, which is just the buoyant force.
  10. Apr 24, 2010 #9
    Your explanation actually made me get it. Thanks :smile:
  11. Apr 24, 2010 #10
    Glad I could help :)
    I only learned that after my physics course, when I saw the derivation with an integral which just calculates this force difference and shows that for a pressure gradient proportional to the depth, the net force is just the amount of water excluded.
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