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Is there a buoyancy force or not?

  1. Mar 3, 2010 #1
    Hi all,
    If i attach a cylinder at the bottom of the water tank such that some of its part is out out of water. So in this case what would be the buoyancy force acting on the cylinder?

    -agtee.
     
  2. jcsd
  3. Mar 3, 2010 #2
    The "buoyancy force" that you are referring to would be upthrust. Archimedes' Principle states that the upthrust acting on an object submerged at a depth in a fluid is equal to the weight of fluid displaced by the object.

    Thus, the upthrust acting on the cylinder would be the weight of fluid displaced by the cylinder (volume of cylinder submerged X density of fluid)
     
  4. Mar 3, 2010 #3
    but in this case if u look how the cylinder is placed there is no water contact at the bottom of the cylinder as it is attached to the bottom of the tank, so no force(as in terms of pressure) form the bottom side ... so the only pressure from water is only on the circumferential face of the cylinder .... that finally nullifies and gives no upward force .. so how actually buoyancy is acting? Please justify this...
     
  5. Mar 3, 2010 #4

    stewartcs

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    None.

    There is no pressure acting on the bottom of the cylinder so there is no upward force. The only pressure acting on the cylinder would be horizontal to the cylinder which results in no upward force.

    CS
     
  6. Mar 3, 2010 #5
    so what are the limitations for the buoyancy force theory?
     
  7. Mar 3, 2010 #6

    Mapes

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    Really??! This is easily falsified experimentally. Just set a lightweight cup in shallow liquid. It will push upward.

    When the cylinder is attached to the bottom surface, there is an energy benefit to its upward movement (assuming the average density of the cylinder is less than the liquid density). This is equivalent to an upward force.

    You might think of the liquid as a microscopic wedge pushing the cylinder off the bottom surface. If the cylinder isn't perfectly attached to the bottom (by suction, for example), there's no way you can keep this "wedge" from getting underneath.
     
  8. Mar 3, 2010 #7
    isnt thr ne macroscopic way of explaining this same phenomena as in the terms of forces acting on the cylinder?
     
  9. Mar 3, 2010 #8

    russ_watters

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    Not if it is sealed to the bottom of the container, it won't.
    That's exactly the situation the OP was after: If you do "perfectly attach" the cylinder to the bottom, then the "wedge" can't get under it.
     
  10. Mar 3, 2010 #9
    so in that case thrs no upward force?
     
  11. Mar 3, 2010 #10

    DaveC426913

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    If it is attached to the bottom, then isn't any upward force moot? How would you measure it? Unless, I guess, it is attached so loosely that any amount of bouyancy will overcome the "glue".
     
  12. Mar 4, 2010 #11
    So now the different case but similar to this one : if there is a hole such that cylinder can just fit into that, so the situation is like lower and the upper part of the cylinder is in the air while middle part is surrounded by water .... now u can measure whether buoyancy is acting or not... what do u think in this case?
     
  13. Mar 4, 2010 #12

    vanesch

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    The point is that when you PLACE the cylinder in the hole or on the bottom, in order to "squeeze out" the water, you have to apply a certain force onto the cylinder, which will result in a slight elastic deformation of the bottom material, deformation which will correspond to a tension in the material corresponding to about the water pressure at that depth. So the fluid pressure is now replaced by a material tension. If you release the cylinder, this tension in the bottom material will exert an upward force (elasticity of the material) which will be equal or even somewhat larger to the upward force one would experience due to the pressure of the liquid if there weren't that bottom.

    However, there will be a slowing-down of the upward movement because at a certain point an upward motion of the cylinder will mean that there needs to be fluid in place, and the surface of influx is the rim of the bottom plate, which is theoretically of 0 thickness. So it might take very long before a small displacement occurs that allows the water to flow underneath the cylinder. Once there is a small film of water, though, the upward motion will not stop.
     
  14. Mar 4, 2010 #13
    Edit This is wrong see my post#29

    -------------------------------------

    Let me start by confirming that there is indeed a bouyancy force given by Archimedes or alternatively as described below.

    There are even real world applications of the case described by the OP.
    Consider a dam impounding a lake with a cylindrical vertical intake tower standing 'fixed' to the bottom and extending above the surface of the lake.
    Both the dam and the tower experience bouyancy forces. Engineers take these into account when designing such structures.

    I can even recall an incident where a Contractor's engineer could not come to terms with this and his river diversion pipes ended floating during a flood at great cost to the project.

    The alternative to Archimedes is to realise that the surface of water in contact with the dam and tower is not a free surface. You can equate the surface energy of this surface to the bouyancy to derive the 'bouyancy force' in much the same way as you can to derive the 'surface tension force'
     
    Last edited: Mar 4, 2010
  15. Mar 4, 2010 #14

    stewartcs

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    You're assuming that pressure is on the bottom of the object. I clearly stated that if no pressure was acting on the bottom then there would be no upward force.

    If their is no pressure acting on the bottom of the object then no upward force can be generated.

    This is easily verified by taking a straw and placing it through a Styrofoam cup and then filling it with water. The straw will not float upward. BTW there are tons of papers on this topic dealing with vertical pipes (risers) in the ocean.

    CS
     
  16. Mar 4, 2010 #15

    stewartcs

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    No, there is no upward force if there is no pressure on the bottom of the cylinder.

    CS
     
  17. Mar 4, 2010 #16

    stewartcs

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    Again, if pressure is not acting on the bottom of the object, no buoyant force will be created (presuming this is a perfectly straight cylinder of course with no other pressure ledges).

    CS
     
  18. Mar 4, 2010 #17

    stewartcs

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    Archimedes principle doesn't apply. If there is no pressure acting on the bottom of this perfectly vertical cylinder then there will be absolutely no buoyant force.

    In all of the examples given, pressure has found some way to get under the cylinder and thus provide and upward force. If it does not, there will be no upward force.

    CS
     
  19. Mar 4, 2010 #18

    stewartcs

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    Actually, no, it's not moot. I deal with very problem almost daily when dealing with the stability of vertical pipes in the ocean. This situation is quite often discussed and even more often misunderstood. People intuitively think that any object in water will weight less. Physics 101 courses don't help by given only the partial picture of what happens (i.e. Archimedes Principle).

    Take a long, vertical, and straight steel pipe and cement it in the ocean floor. How much tension would you have to apply to support just the weight of pipe? Intuition would tell you, based on Archimedes Principle, just the wet weight of the steel pipe. However, this is not the case since no pressure is acting on the bottom of the pipe (it is sealed). The tension required would be equal to the air weight of the steel pipe.

    CS
     
  20. Mar 4, 2010 #19
    Edit This is wrong see my post#29
    --------------------------------------
    Take a sealed cylinder with flanged flat ends, longer than the depth of water.

    Float it on the surface and tow it out horizontally.

    Force (yes you will have to ) it to the bottom where you have a preprepared flat steel plate.

    With suitable handling equipemnt turn it to the vertical.

    Bolt the end flange to to the plate and tighten down.

    It will be a cylinder, fixed to the bottom, sticking out of the water as prescibed with no water under it and therefore no water pressure under it.

    Now release the bolts.

    What happens?

    If the plate were the measuring table of a weighbridge, would it measure the same weight as a similar weighbridge on land?
     
    Last edited: Mar 4, 2010
  21. Mar 4, 2010 #20

    Mapes

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    I made a diagram with similar reasoning to Studiot. Consider an object with a strain gauge attached, rigidly attached to the container bottom (welded or epoxied, say, so that no liquid can get underneath). I've marked the downward water pressure that acts on the top face of the object; of course, water pressure is exerted on the other faces also.

    Now attach an empty cylinder to the object (rigidly by welding or epoxy, so that no water can get underneath). The downward water pressure on the support has now been removed. The support and strain gauge will therefore elongate due to this change in stress.

    Does this not represent and measure the buoyant force of the empty cylinder?

    EDIT: This represents just one possible configuration; please see my post #23 below.
     

    Attached Files:

    Last edited: Mar 4, 2010
  22. Mar 4, 2010 #21
    Strickly speaking, you have not fully specified the questions properly. My guess is that you are implying that the cylinder is vertically oriented. In idealized cases, a perfectly vertical cylinder will not have buoyancy force if water can not get under the bottom. Ideal cases can be the example of putting a straw through a cup, or a perfectly rigid container with a perfect seal between the container and cylinder.

    However, let's consider non-ideal cases.

    If the cylinder is not perfectly vertical then there will be some buoancy force acting on the sides. The force will be proportional to sine of the angle from vertical.

    In the vertical case, the surrounding water places a force on the bottom of the ocean/container which is different from what the cylinder is placing on the bottom. Depending on the material of the floor, there can be different responses to stress forces acting on the floor. The cylinder will transmit the atmospheric pressure, plus it's weight per unit area down onto the bottom/floor, while the surrounding water will transmit the atmospheric pressure plus the water depth pressure down on to the bottom. Usually, the pressure on the floor is greater where the cylinder is because the cylinder's density is greater than that of water. There will be stresses and possible consequences of this. If the floor is solid, then there really is no buoyancy force, but if it has some fluid properties, you can argue that the water pressure is partially transmitted through the floor and back onto the bottom of the buoy. In this case, there is some buoyancy force, in a sense. In the case of an ocean, one would need to ask about the material properties of the bottom. Sand saturated with water might behave differently than a solid rock-bed.

    It's also interesting to consider the case where the cylinder top is submerged. In this case there will be a downward force exerted on the top surface.
     
  23. Mar 4, 2010 #22

    DaveC426913

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    Got it. There should be a way of arranging a strain gauge on the bottom of the cylinder so that, if it had a bouyant tendency, it could be measured. All without letting water under the cylinder.

    I guess what you've got is a vacuum chamber underneath with a strain gauge inside.
     
  24. Mar 4, 2010 #23

    Mapes

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    OK, I think I might be able to resolve these opposing views, again considering the energy of the system as a whole. Recall that a force corresponds to a reduction in system energy for a given infinitesimal displacement:

    [tex]\bold{F_i}=-\left(\frac{dE}{dx_i}\right)\bold{i}[/tex]

    If there is no change in energy, there is no driving force. In my diagram above, there is a lateral contraction of the support as it elongates, so the potential energy of the liquid is decreased as its upper level drops. A buoyancy force is measured.

    Studiot's real-life example probably corresponds to my diagram, where a non-rigid attachment connects a hollow object to the basin bottom. In this case, it would be necessary to account for a buoyancy force.

    If the support were absent, the container bottom rigid, and the cylinder rigidly attached with no liquid intrusion underneath, it makes no sense to talk about a buoyant force, since it could not be measured even in principle.

    If liquid intrusion under the cylinder is allowed, the cylinder will be pushed up due to the lowering of liquid potential energy as its upper level drops.

    When a straw is pushed through a cup bottom, there is no energy benefit associated with moving the straw up or down. Thus, there is no buoyancy force. This probably corresponds to the real-life design experiences contributed by stewartcs.

    Does this seem reasonable to everybody?
     
  25. Mar 4, 2010 #24

    DaveC426913

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    Consider my modification to add a chamber underneath that liquid cannot penetrate but has room for a strain guage.

    Now that I think about it, the chamber should be filled - not with vacuum, but with air at 1 atmo - removing any non-water forces from the equation.


    [ EDIT ] No, this cannot work. The pressure differential across the water-air or water-vacuum seal will result in friction forces that cannot be ignored.

    Perhaps, but the initial force will start at zero, since initially there is no water underneath. If done carefully enough, the duration of this zero force condition could be extended arbitrarily.
     
    Last edited: Mar 4, 2010
  26. Mar 4, 2010 #25

    DaveC426913

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    This is the crux of the question being asked. Your response is, no there is no vertical bouyant force.
     
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