Floating Table (With Buckets)

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  • #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,
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  • #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.

If you think about it, the buckets are actually holding down the table.

Are they?
 
  • #3
Demystifier
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Does anybody have any idea how you would conduct calculations on doing this?
4 M_bucket >= M_table
where M, of course, is the mass.

By the way, this thread should be moved to General Physics.
 
  • #4
Doc Al
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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.
Instead of buckets, imagine there were people pulling down on the ropes (if the table were bigger). Would that make more sense?

Does anybody have any idea how you would conduct calculations on doing this?
Treat it as any other static equilibrium problem.


I think what you have here is an optical delusion, a little piece of performance art made at the expense of physicists/engineers.
Why do you think that?
 
  • #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.
 
  • #6
davenn
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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.

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
 
  • #7
Doc Al
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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.
Have you heard of a bosun's chair?
bosuns1.gif
 
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  • #8
Vanadium 50
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Think about what bathroom scales underneath the buckets would read.
 
  • #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" ;)

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  • #11
SteamKing
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Have you heard of a bosun's chair?
bosuns1.gif

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.
 
  • #12
Doc Al
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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.
I think you mean tension in each rope, not static friction.

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
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?)
 
  • #13
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?)

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
 
  • #14
Doc Al
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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.
The point of the bosun's chair is that you can hoist yourself up. You don't need someone else.

If you think the buckets are holding the tabletop down, how are you able to push on a rope?
? The ropes just pull, like always.

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

Unless you are claiming that gravity's hours have been cut back and it works only part time now.
24/7 baby!
 
  • #15
Doc Al
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The buckets and table are all going to have to meet at the same point in the air.
Not sure what that means.

Answer my question: What forces act on the "buckets + table" system?
 
  • #16
A.T.
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But the chair doesn't work if it's not tied off or if someone isn't hoisting you up or down.
The chair works as shown in the picture, as long as the man is heavier than the chair.
 
  • #17
sophiecentaur
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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.

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.
 
  • #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.
 
  • #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.
 
  • #20
sophiecentaur
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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.

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.
 
  • #21
nasu
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You don't even need a chair.
You can pull yourself up with a rope thrown over a tree branch. You just make a little loop at one end so you put for foot through it and pull the other end of the rope.

Here is a more sophisticated, "modern" contraption:
She has some extra pulleys to reduce the force (by pulling more rope, in exchange, as can be seen).

Or even a bicycle::smile:
 
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  • #22
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If you start off by imagining that the buckets are not in contact with the table, but static floating slightly over the table surface, then you know that the weight of the four buckets is close to the weight of the table... if you know the angle the ropes make from vertical as they run from the pulleys to the table, you could figure the difference.

If the buckets are too light, the table will fall, if the buckets are just the right weight, the table and buckets will hang without contact, and if the buckets are heavier than that, the whole thing will continue to hang with increasing tension in the ropes... until something breaks.

The critical thing looks like finding the rope tension that suspends the table statically without bucket contact, beyond which the whole thing will hang statically with increasing force between the buckets and table, and increasing rope tension constant throughout their lengths (Vanadium 50's scale under the buckets question).

To treat it mathematically, without motion and the buckets not in contact with the table, the ropes upward vertical component equals the downward vertical component of the weight of the table and buckets. Maybe what you want is a relationship of the force between the buckets and table pressing together, and the increase in tension in the ropes, as a function of bucket weight?

Something like:

Bucket weight = Wb
Table weight = Wt
Rope tension = T
Force between buckets and table = F
Trig adjustment for rope angles = ta

So
F= ta2T(4Wb+taWt) or something like that...?
 
  • #23
SteamKing
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The point of the bosun's chair is that you can hoist yourself up. You don't need someone else.

'Someone' is indefinite. It can mean the person in the bosun's chair or a second party.

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

Now, you are being a sophist. Yes, I can stand on the floor and walk without floating in the air. However, last time I checked, table tops did not float above the floor and have to be restrained by a contraption like is shown in the photo.
 
  • #24
Doc Al
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'Someone' is indefinite. It can mean the person in the bosun's chair or a second party.
The interesting (and relevant) option is when the person sitting in the chair pulls himself up.

Now, you are being a sophist. Yes, I can stand on the floor and walk without floating in the air.
My point was that as long are there is a normal force pressing you to the ground--or the buckets to the table--there is no need for any mechanical connection. And that is easily arranged.

However, last time I checked, table tops did not float above the floor and have to be restrained by a contraption like is shown in the photo.
I trust you don't really think the table is 'floating in the air'. It's hanging from ropes! :smile:
 
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  • #25
A.T.
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'Someone' is indefinite. It can mean the person in the bosun's chair or...
... a heavy bucket on the bosun's chair.
 

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