Tricking Archimede's principle

  • Thread starter jumpjack
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  • #1
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What would it happen if...

...you get half a wooden sphere, put it into a bucket filled with water, making the flat part of the half-sphere touching the bucket bottom?

Will the half-sphere come up floating?
Maybe not, 'cause water has no way to push it up? :confused:

A video woukd be cool.
 

Answers and Replies

  • #2
stewartcs
Science Advisor
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What would it happen if...

...you get half a wooden sphere, put it into a bucket filled with water, making the flat part of the half-sphere touching the bucket bottom?

Will the half-sphere come up floating?
Maybe not, 'cause water has no way to push it up? :confused:

A video woukd be cool.

If there is no pressure acting on the bottom of the half-sphere it will not float.

However, with this type of experiment you'll likely find that there isn't a practical way to remove the water from the underneath side of the half-sphere once you submerge it in the bucket.

CS
 
  • #3
mathman
Science Advisor
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The situation you described is extremely unstable. The slightest jitter of the half-sphere would allow water to get underneath and push it up.
 
  • #4
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If there is no pressure acting on the bottom of the half-sphere it will not float.

However, with this type of experiment you'll likely find that there isn't a practical way to remove the water from the underneath side of the half-sphere once you submerge it in the bucket.

CS
then if I place it on the bucket bottom and THEN I fill the bucket, it should stay there...?

What would be more suitable? a less-than-half sphere, or a... gothic-arc-shaped-wood-piece? :smile: (It's designed to carry all the weight it receives from above...)
 
  • #5
stewartcs
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then if I place it on the bucket bottom and THEN I fill the bucket, it should stay there...?

What would be more suitable? a less-than-half sphere, or a... gothic-arc-shaped-wood-piece? :smile: (It's designed to carry all the weight it receives from above...)

then if I place it on the bucket bottom and THEN I fill the bucket, it should stay there...?

If the water is prevent from entering the underneath side (e.g. a seal) then yes.

What would be more suitable? a less-than-half sphere, or a... gothic-arc-shaped-wood-piece? :smile: (It's designed to carry all the weight it receives from above...)

I don't know what a gothic-arc-shaped-wood-piece looks like. However, stability of floating bodies is determined the same way.

Take a look here for more information on that: http://www.coastal.udel.edu/faculty/jpuleo/CIEG305/stability_floating_body.pdf

CS
 
  • #6
ideasrule
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http://en.wikipedia.org/wiki/Suction_cup" [Broken]?
 
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  • #7
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gothic arc:

arco.jpg


And what about a swing-shaped object?
lift.jpg

It has more surface available for pressure from above than it has for "floating force" coming from below...
Would it flow or sink? (if static).
 
  • #8
stewartcs
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gothic arc:

arco.jpg


And what about a swing-shaped object?
lift.jpg

It has more surface available for pressure from above than it has for "floating force" coming from below...
Would it flow or sink? (if static).

The resultant force is what determines if it will float or sink (that and the weight of the object of course). Archimedes Principle will work just fine in order to determine if the object will float or not when placed in water.

CS
 
  • #9
215
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The resultant force is what determines if it will float or sink (that and the weight of the object of course). Archimedes Principle will work just fine in order to determine if the object will float or not when placed in water.

CS
yes, but I can't understand WHICH forces try balancing in archimedes' principle: gravity and...?
I don't think it is the attraction among H2O molecules (which causes a hole in the water to close by itself) can push over an object submerged into the water, trying to "expell" it... unless you're in outer space(*).
No, I think archimede's principle depends on gravity: but if gravity only pulls down, how do H2O molecules act on a body to "reverse" gravity effect?

(*)
what does it happen to half-a-sphere submerged in water if there is no gravity? I guess a wooden sphere would remain inside the water sphere (!) where you put it. But if you have a wooden HALF sphere... will it start moving?!? :uhh:
 
  • #10
stewartcs
Science Advisor
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yes, but I can't understand WHICH forces try balancing in archimedes' principle: gravity and...?

The force generated by the hydrostatic pressure acting over the surface area of the object.

Draw a FBD and sum the forces. If there is net upward force the object will rise (until it reaches the surface), if the net force is zero the object will float, if it is negative it will sink (until it reaches the bottom).

CS
 

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