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Floating Table (With Buckets)

  1. Feb 17, 2014 #1
    Hello Physics Forums, Deployment here. After browsing the internet I came across this photo..

    ieIf2NQ.jpg

    I have been pondering and pondering on how this is actually possible. If you think about it, the buckets are actually holding down the table.

    Does anybody have any idea how you would conduct calculations on doing this?

    Thanks,
    - Deployment​
     
  2. jcsd
  3. Feb 18, 2014 #2

    SteamKing

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    I think what you have here is an optical delusion, a little piece of performance art made at the expense of physicists/engineers.

    Are they?
     
  4. Feb 18, 2014 #3

    Demystifier

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    4 M_bucket >= M_table
    where M, of course, is the mass.

    By the way, this thread should be moved to General Physics.
     
  5. Feb 18, 2014 #4

    Doc Al

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    Instead of buckets, imagine there were people pulling down on the ropes (if the table were bigger). Would that make more sense?

    Treat it as any other static equilibrium problem.


    Why do you think that?
     
  6. Feb 18, 2014 #5

    SteamKing

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    The table is either stuck to the wall or it's stuck to the buckets. It's impossible to tell which just by looking at this photo.
     
  7. Feb 18, 2014 #6

    davenn

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    dont think so

    I can imagine an upwards force from the table against the bottom of the buckets and the mass of the buckets ( including anything they contain ... maybe some water) producing an equal force down against the table top.

    I dont see anything implausible in the scenario

    Dave
     
  8. Feb 18, 2014 #7

    Doc Al

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    Have you heard of a bosun's chair?
    bosuns1.gif
     
  9. Feb 18, 2014 #8

    Vanadium 50

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    Think about what bathroom scales underneath the buckets would read.
     
  10. Feb 18, 2014 #9

    anorlunda

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    I don't see anything odd at all. What did you expect to happen?
     
  11. Feb 18, 2014 #10
    Firstly, sorry for placing this in the wrong section..I am just learning my way around here.

    Secondly, this is 100% possible, with no tricks such as the table being attached to the wall, etc. We are currently doing this in physics class, and I was wondering if the geniuses at Physics Forums had any ideas as to where I should start.

    I know that equilibrium is where I need to focus, and the idea that the same amount of static friction needs to be pulled on each rope. Im just trying to put together a road map of equations so I can figure out the answer

    Today I will be given the values for..
    * The weight of the table
    * The weight of the buckets
    * The angle of the strings

    Then I will simply need to calculate how much water must be place into each bucket in order for the table to "float" ;)

    - Deployment
     
  12. Feb 18, 2014 #11

    SteamKing

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    Yes, I've heard of a bosun's chair. But the chair doesn't work if it's not tied off or if someone isn't hoisting you up or down.

    If you think the buckets are holding the tabletop down, how are you able to push on a rope? All I see are four sheaves, each with a single line running thru it, with one end tied to the table top and the other end tied to a bucket handle. If there is no mechanical connection between the buckets and the table top (bolts, glue, suction cups, whatever), the table top must be attached to the wall, which connection is not visible due to the viewpoint of the camera.

    Unless you are claiming that gravity's hours have been cut back and it works only part time now.
     
  13. Feb 18, 2014 #12

    Doc Al

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    I think you mean tension in each rope, not static friction.

    Sounds good.

    As far as equations go, what are the conditions for equilibrium? (Hint: Consider the "buckets + table" as a single system. What forces act on it?)
     
  14. Feb 18, 2014 #13
    The buckets and table are all going to have to meet at the same point in the air. By putting the same amount of weight in each bucket, the buckets are taken care of and will all be equal. The question is how much weight.

    I have the logic and can explain it to anyone, the equations are just what is holding me up at the moment.

    Hopefully class teaches me a little bit more today..or someone on here kills it with knowledge!

    - Deployment
     
  15. Feb 18, 2014 #14

    Doc Al

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    The point of the bosun's chair is that you can hoist yourself up. You don't need someone else.

    ? The ropes just pull, like always.

    Is there a mechanical connection between you and the floor? Yet you can walk without floating into the air!

    24/7 baby!
     
  16. Feb 18, 2014 #15

    Doc Al

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    Not sure what that means.

    Answer my question: What forces act on the "buckets + table" system?
     
  17. Feb 18, 2014 #16

    A.T.

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    The chair works as shown in the picture, as long as the man is heavier than the chair.
     
  18. Feb 18, 2014 #17

    sophiecentaur

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    It's perfectly possible to pull yourself up with a Bosun's Chair. You do need strong arms tho' and it's much easier with a beefy mate at the bottom of the mast. There is no difference between a Bosun's chair and a mass on the end of a loop which has been passed over a pulley. The passenger can change the length of the rope (i.e. height) by pulling in or letting out one of the ends of rope.

    To be honest, there is a bit of cognitive dissonance in that picture. Despite my rational approach to it, which shows that it's perfectly possible, I still find it a bit magical. But isn't that what all conjuring tricks rely on?

    In many ways, you could say that the buckets are 'holding the table down' against the tension in the rope. If there were four, bucket sized holes in the table, it would go up, wouldn't it??! (The tensions in the ropes would change when the holes appeared, of course.
     
  19. Feb 18, 2014 #18

    sophiecentaur

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    There is also the matter of stability to consider. The arrangement even 'appears to be' in unstable equilibrium but there are those triangular structures with the strings under tension and a rigid table so there will be no change of shape even with a range of asymmetrical weights of bucket + water in the buckets. It could fail in the end, with big enough differences in weight.
     
  20. Feb 18, 2014 #19

    jbriggs444

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    With four buckets and four [pairs of] ropes, the system is over-constrained. The set-up process may involve careful measurement of ropes and positioning of buckets to ensure that none of the ropes go slack.

    It might be simpler to analyze with just three buckets and three [pairs of] ropes.
     
  21. Feb 18, 2014 #20

    sophiecentaur

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    That's an excellent idea - a la three legged stool problem. Doing it that way, you could assume unstretchable ropes and always get all three buckets in contact with the table.
     
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