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Thread: what makes buoyancy?

  1. #1 what makes buoyancy? 
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    how does buoyant force result from the difference in pressure between the surface of the cylinder in the fluid and its base?I can't imagine this at all.

    We also study that the buoyancy is the reaction of the fluid to the object pushing on it

    I can't relate all of this together...

    *water pushes also on the bottom surface of the object in the downwards direction as well as upward direction :right?so what makes buoyancy?

    http://img696.imageshack.us/img696/3654/unled22rg.jpg
    (ignore that the tank is full of water)

    you would tell me that there's not a downward force at the base of this suspended object,there's only an upward force...so why??


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  3. #2  
    Forum Professor wallaby's Avatar
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    That's right, the water pushes on the bottom surface of the container. But since this water has no where to go there is a reaction force, imparted on the water in contact with the surface, by the container and continued through the contact of water molecules, although this reaction force is diminished as you get closer to the surface.

    Quote Originally Posted by Misr
    how does buoyant force result from the difference in pressure between the surface of the cylinder in the fluid and its base?
    An object in a fluid is pulled down by gravity, pushed down by the weight of the fluid on top of it and pushed back up by the resistance of the fluid below it to compression. The force acting per unit area on the object is of course the pressure of the fluid and so the object will be buoyant when the pressure at the bottom can balance the pressure on the top of the object, as well as the weight of the object. In other words the difference in the pressure must be equal to the weight of the object.


    Last edited by wallaby; October 30th, 2011 at 10:15 PM. Reason: Commas and clarification
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    Quite frankly, I don't understand the idea of buoyancy involving the weight of the fluid above and below an object. Isn't it as simple as the gravitational force of the liquid displaced by the object? An object with a volume of one cubic foot displaces the same volume of water whose gravitational force would be about 62 pounds. The weight of the object determines the resultant force and the vertical movement of the object. There's special considerations for submarines, humans, etc that are compressible (and thus, whose volume can change).
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    This may help. The density of an irregular solid can be found as follows.

    Experiment. Density of irregular solid.

    Method. Weigh a sample of metal M on a balance. Partly fill a measuring cylinder with water and record the level. Lower the metal gently by thread so that it is completely immersed in the water. Observe the new level in the measuring cylinder. The difference in volume is then then the volume of M. A Eureka can is useful if the sample is too big for the measuring cylinder.
    Calculations. Calculate the the density from density =mass/volume.
    .
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    Typically, a drop of water feels pressure from above and below in equal amounts. Ordinarily the 2 balanced pressures are equal to the weight of the water above it (it doesn't really matter how much water there is below it), and cancel out.

    Now, suppose this drop of water is located underneath a hollow sphere, like a beach ball that has been pushed underwater. Now, it's feeling the same force pushing up from below it as it would feel if it had the normal amount of water above it for that depth, but...... the normal amount of water above it is not present to push it down. The downward force on our drop of water is only equal to the quantity of water that actually is located above it, not the amount that should be there.

    Since the two forces no longer balance, that drop of water is pushed upward by the difference.
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    Quote Originally Posted by jrmonroe View Post
    Quite frankly, I don't understand the idea of buoyancy involving the weight of the fluid above and below an object. Isn't it as simple as the gravitational force of the liquid displaced by the object? An object with a volume of one cubic foot displaces the same volume of water whose gravitational force would be about 62 pounds. The weight of the object determines the resultant force and the vertical movement of the object. There's special considerations for submarines, humans, etc that are compressible (and thus, whose volume can change).
    Archimedes' Principle provides a quantitative description of buoyancy, it says nothing of how buoyancy is achieved.
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  8. #7  
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    Thought experiment: Imagine you take a beach ball and submerge it half way into the water. Now imagine a teleportation machine teleports the beach ball away instantly, leaving a cavity in the water where the ball had been. Clearly that cavity is going to fill in. Why?
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    Quote Originally Posted by Dave Wilson View Post
    This may help. The density of an irregular solid can be found as follows.

    Experiment. Density of irregular solid.

    Method. Weigh a sample of metal M on a balance. Partly fill a measuring cylinder with water and record the level. Lower the metal gently by thread so that it is completely immersed in the water. Observe the new level in the measuring cylinder. The difference in volume is then then the volume of M. A Eureka can is useful if the sample is too big for the measuring cylinder.
    Calculations. Calculate the the density from density =mass/volume.
    Now stop running naked through the streets. Balloons exhibit buoyancy in air, so would it be worthwhile to mount solar panels on same to take them above clouds?

    No.
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  10. #9  
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    Quote Originally Posted by The Finger Prince View Post
    Quote Originally Posted by Dave Wilson View Post
    This may help. The density of an irregular solid can be found as follows.

    Experiment. Density of irregular solid.

    Method. Weigh a sample of metal M on a balance. Partly fill a measuring cylinder with water and record the level. Lower the metal gently by thread so that it is completely immersed in the water. Observe the new level in the measuring cylinder. The difference in volume is then then the volume of M. A Eureka can is useful if the sample is too big for the measuring cylinder.
    Calculations. Calculate the the density from density =mass/volume.
    Now stop running naked through the streets. Balloons exhibit buoyancy in air, so would it be worthwhile to mount solar panels on same to take them above clouds?

    No.
    Nice one.
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