Face
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Face
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April 18th, 2011 at 4:07:34 PM permalink
Here we have a stand.
On this stand there is a tank. The tank weighs 150 units.
In the tank there is water. The water weighs 100 units.
In the water there are rocks. The rocks weigh 50 units.
There are now 300 units of weight pressing down on the stand.
Here I have a fish. The fish weighs 5 units.
If I place the fish in the water, and the fish stays suspended off the bottom, how many units of weight are now pressing down on the stand?
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odiousgambit
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April 18th, 2011 at 4:13:59 PM permalink
305 units, I would say the fish's buoyancy is immaterial
the next time Dame Fortune toys with your heart, your soul and your wallet, raise your glass and praise her thus: “Thanks for nothing, you cold-hearted, evil, damnable, nefarious, low-life, malicious monster from Hell!” She is, after all, stone deaf. ... Arnold Snyder
AZDuffman
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April 18th, 2011 at 4:14:54 PM permalink
Quote: Face

Here we have a stand.
On this stand there is a tank. The tank weighs 150 units.
In the tank there is water. The water weighs 100 units.
In the water there are rocks. The rocks weigh 50 units.
There are now 300 units of weight pressing down on the stand.
Here I have a fish. The fish weighs 5 units.
If I place the fish in the water, and the fish stays suspended off the bottom, how many units of weight are now pressing down on the stand?



It is impossible not to have the fish pushing down. The fish displaces water, adding weight to the tank.
All animals are equal, but some are more equal than others
Face
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Face
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April 18th, 2011 at 4:35:40 PM permalink
305 was my original assumption. If 5 units of mass is added, then it has to be there, right?

But then I thought about my livewell. A cooler full of water has weight. Fill it with 20 crappie and I couldn't notice a difference. Of course a few pounds of fish might not be noticible when struggling with a 100lb box, but it made me second guess.

And then I thought about the 'pushing down' principle. Was it really? If it was pushing down, wouldn't it sink? Water would just move out of the way, would it not?

If a ballon, filled with just enough helium to float level, floats into your window at home, does your house get heavier?

Maybe I'm just having a dumb moment. =P I keep going back and forth on this.
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Doc
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April 18th, 2011 at 4:55:32 PM permalink
Quote: Face

... If a ballon, filled with just enough helium to float level, floats into your window at home, does your house get heavier? ...


For most of your comments, I think you just need to hire some consulting services from Archimedes. For this specific question, I'll assume you are talking about the house and everything it contains at the moment. The entry of the helium balloon through the window would tend to make the house/contents heavier, except for the fact that I suspect an equal volume of air would be pushed back out of the window to make room for the balloon. Since the balloon floats level, the departing air has the same mass/weight as the balloon and helium, so there is no change in the total weight. Got that?
Face
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Face
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April 18th, 2011 at 5:12:22 PM permalink
Quote: Doc

For most of your comments, I think you just need to hire some consulting services from Archimedes. For this specific question, I'll assume you are talking about the house and everything it contains at the moment. The entry of the helium balloon through the window would tend to make the house/contents heavier, except for the fact that I suspect an equal volume of air would be pushed back out of the window to make room for the balloon. Since the balloon floats level, the departing air has the same mass/weight as the balloon and helium, so there is no change in the total weight. Got that?



I think Archimedes would blow me off as a kook ;). You're explanation made me think, Doc. Maybe the weight of the air in the volume of said balloon would equal the weight of the balloon itself, and make your example true. But how about this...

We borrow Mythbuster's giant hanger and hypothetically place a scale under it. It weighs X
I walk inside. It now weighs X+200lbs.
I am handed a giant balloon with a helium tank attached.
A balloon of such size weighs 150lbs, the tank weighs 50lbs and the Helium has positive weight.
All told we have X+200+150+50+helium.
If I use the tank to inflate said balloon until the balloon, the tank and myself are now floating just off the ground, wouldn't the scale simply read X, with the air which was displaced not possibly weighing 400lbs+weight of helium?
This concept is not unlike a fish's swim bladder inflating with air and causing it's buoyancy, which is why I can't say 305 units for sure. Any thoughts?
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Ayecarumba
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April 18th, 2011 at 5:34:15 PM permalink
Yes, the scale under the hanger would only indicate "X". The displacement of the rest of the weight by the helium filled balloon would not register (but the helium filled ballon would still have mass).
Simplicity is the ultimate sophistication - Leonardo da Vinci
Face
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Face
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April 18th, 2011 at 5:38:26 PM permalink
Quote: Ayecarumba

Yes, the scale under the hanger would only indicate "X". The displacement of the rest of the weight by the helium filled balloon would not register (but the helium filled ballon would still have mass).



So, if we were to apply this concept to the first post with the 'fish in a tank' example, then 300 units would be the force on the stand? =D
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thecesspit
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April 18th, 2011 at 5:39:52 PM permalink
Only if the 5lb of water displaced by the fish is spilled out of the tank... (like the air in the balloon example is forced out of the house).
"Then you can admire the real gambler, who has neither eaten, slept, thought nor lived, he has so smarted under the scourge of his martingale, so suffered on the rack of his desire for a coup at trente-et-quarante" - Honore de Balzac, 1829
Ayecarumba
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April 18th, 2011 at 5:48:06 PM permalink
Quote: Face

If it was pushing down, wouldn't it sink? Water would just move out of the way, would it not?



Back to the tank, if you were to weigh the whole thing, it would be 305. Just because something floats in a medium, doesn't mean that its mass is neutral. Think about the a giant livewell, with a super sized crappie shaped airtight boxes made of depleted uranium. As you add each one to the livewell, you will note that an equal amount of water is displaced to keep them afloat. You would definitely notice the additional weight (unless the displaced water was allowed to flow away)
Simplicity is the ultimate sophistication - Leonardo da Vinci

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