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What will happen when a body is equally dense as the liquid?

  1. Oct 27, 2013 #1
    I am studying Floatation in school and I have some doubts. I get that if an object is denser than the liquid its placed on, it will sink, and if it is less dense then it floats. But what happens when the density of both (the object and the liquid) are same? Will it float or sink?

    My notes say that if it has a lesser density, then it float PARTIALLY &
    if it has an equal density to that of the liquid it will FLOAT.

    But how and why? I searched the net and found different explanations that confused me even more. Hope you can help.

    KUDOS. :D
     
  2. jcsd
  3. Oct 27, 2013 #2

    CWatters

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    Read up on "Displacement". For example ships.
     
  4. Oct 27, 2013 #3
    If the liquid is water, and the body has the same density as water, then it will most likely float because of the surface tension of water caused by hydrogen bonds.
     
  5. Oct 27, 2013 #4
    If an object is exactly the same density as the liquid, it will not move up or down. It will just stay right where it is (unless it is pushed around by water currents). If you put it on the surface, it will remain on the surface [EDIT: see CWatters post below for clarification]. If you push it under the water, it will stay under the water, without moving up or down.

    It's not so easy to achieve neutral buoyancy exactly. Fish and submarines do it by having an internal container that they can pump air and water into or out of in order to change their density.
     
    Last edited: Oct 27, 2013
  6. Oct 27, 2013 #5

    CWatters

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    Perhaps best to clarify that last line.. It will actually sink into the water so that no part of the object is above the surface.
     
  7. Oct 27, 2013 #6

    CWatters

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    There are three cases..

    If an object is less dense than water it will sink into the water until the mass of water displaced equals the mass of the object. If pushed further under it will float back up.

    If the object is of equal density it will sink into the water so that no part of the object is above the surface. It may or may not "float" just but entirely below the surface. As eigenperson said, if pushed down it might stay at the new depth but that's tricky.

    If the object is of greater density it will sink.
     
  8. Oct 27, 2013 #7
    Why is it tricky? If pushed down it will stay at the new depth unless acted on by another force. The force of water pressure would be equal on all sides would it not? Although there is less pressure exerted on it near the surface than there would be at greater depth. But that could be negligible depending on the size of the object. Also at such negligible magnitude of forces doesn't gravity still play a role? On second thought maybe you're right, this could get tricky ;) But tricky is interesting.:approve:
     
  9. Oct 28, 2013 #8

    CWatters

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    What I meant was it's hard to achieve exactly neutral buoyancy in the first place. Any slight error in the density and the object starts to float or sinks. If the object is slightly compressible then changes in water pressure may change it's volume slightly and hence it's density. Even temperature changes may effect it. This effect is exploited here.

    http://www.howstuffworks.com/question663.htm

    As the temperature changes a bubble will transition through the neutral density point and either float or sink.
     
    Last edited: Oct 28, 2013
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