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Why does something float?

  1. Dec 20, 2011 #1
    I know that for something to float on water it has to displace an amount of water equal to its weight or more than that . But why is this so ?
     
  2. jcsd
  3. Dec 20, 2011 #2
  4. Dec 20, 2011 #3

    Doc Al

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    The weight of water displaced will equal the weight of the floating object.
    Are you familiar with Archimedes' principle? The upward buoyant force on an object due to the pressure of the water equals the weight of the displaced water. If it's floating, the net force on it must be zero, so the upward buoyant force must balance the downward weight of the object.
     
  5. Dec 20, 2011 #4
    Oh I see. There was one website that said if a light object displaces a large amount of water in relation to its weight , it will float .

    Yes I do know what it is but I have not learnt about forces yet . Does this mean I have to have a basic understanding of forces to understand Archimedes principle ?
     
  6. Dec 20, 2011 #5
    Thank you travis_king!
     
  7. Dec 20, 2011 #6
    The magnitude of that force is proportional to the difference in the pressure between the top and the bottom of the column, and is also equivalent to the weight of the fluid that would otherwise occupy the column. For this reason, an object whose density is greater than that of the fluid in which it is submerged tends to sink.


    I copied the above from Wikipedia and I understand the first part but not the second part. Why is the force equivalent to the the weight of fluid that would otherwise occupy the column ??
     
  8. Dec 20, 2011 #7

    Doc Al

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    I don't quite understand that statement. A light object will displace a small volume of water (compared to the volume of the object), but that's all that's needed for it to float.

    You need to know a little bit about forces, since floating and Archimedes' principle involves the force of gravity (weight) and the buoyant force.
     
  9. Dec 20, 2011 #8

    Doc Al

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    Read this: Buoyancy
     
  10. Dec 20, 2011 #9
    Ok so basically the first experiment is trying to tell
    Me that as long as two objects have the same volume they will experience the Same amount f pressure and as since there is same amount of pressure it means that the buoyant force up against the water ball is the same as the buoyant force up against the actual ball of the same volume and hence when the same volume ball replaces the water ball it will experience ye same internal pressure but ends up displaces an amount of water which is of the same volume as it is . Did I understand it correctly ?
     
  11. Dec 20, 2011 #10
    Oh wait it's not the volume but the weight it seems like I still cannot understand it
     
  12. Dec 20, 2011 #11

    Doc Al

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    Sounds like you've got it. Let me restate it:

    The force of the water on the 'ball' (the buoyant force) is due to the pressure of the water outside the ball and does not depend on what's in the ball.
    The first diagram shows that that buoyant force must equal the weight of an equal volume of water (the water ball example).
    The second diagram shows that the same upward buoyant force will be exerted on any equal-size 'ball' regardless of what it's made of.
     
  13. Dec 20, 2011 #12
    All posters are saying the same thing in many different ways. So, I'll also give it a try.

    (Object's weight in air) - (Object's weight in water) = (Weight of displaced water) --> by Archimedes

    For a floating object,

    (Object's weight in air) = (Weight of displaced water)

    This means (Object's weight in water) must be zero. If the weight is zero, it must be floating.
    I once calculated, under some conditions, an object can stand still under water (not falling to the ocean floor). I think submarines use this conditions also.
     
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