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Author Topic: Weighing Water While under Water  (Read 4821 times)

Offline ukmicky

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Weighing Water While under Water
« on: 25/04/2007 18:31:29 »
I was asked this and know the answer but cant think of an easy to understand way to explain it to my 10 Y old niece .

If i were to drop a set of scales in a swimming pool or drop it into the sea why wont it record the weight of the water pushing down on it from above.
« Last Edit: 08/05/2007 11:08:40 by BenV »


 

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Re: Weighing Water While under Water
« Reply #1 on: 25/04/2007 18:48:57 »
The sea presses down on the platform of the scales (the bit you stand on) but it also presses up from underneath. These 2 forces cancel out.
 

Offline ukmicky

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Re: Weighing Water While under Water
« Reply #2 on: 25/04/2007 18:55:56 »
That was what i said ,and her reply was "but if i dropped your bathroom scales in the water their is nothing for the water to push upon " In other words the water cant get underneath the standing surface to push it up.

Ok scales have holes but what if it were sealed.


Dont you just hate it when they sit there think for a minute and complicate things.
« Last Edit: 25/04/2007 19:08:02 by ukmicky »
 

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Re: Weighing Water While under Water
« Reply #3 on: 25/04/2007 19:39:11 »
That was what i said ,and her reply was "but if i dropped your bathroom scales in the water their is nothing for the water to push upon " In other words the water cant get underneath the standing surface to push it up.

Ok scales have holes but what if it were sealed.


Dont you just hate it when they sit there think for a minute and complicate things.

You cannot really have sealed scales.

The problem is that what you describe with regard to water does not only apply to water, it also applies to air, which also has substantial pressure.  If you seal the scales, then the scales become susceptible to variations in air pressure, and you no longer have a set of scales but a barometer.
« Last Edit: 25/04/2007 19:41:57 by another_someone »
 

Offline Bass

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Re: Weighing Water While under Water
« Reply #4 on: 25/04/2007 20:16:21 »
So this begs the question- do changes in air pressure affect your weight as measured by the scale?  I would think not- maybe this has something to do with air/water pressure acting in all directions while your finite (some are more finite than others) body only exerts downward pressure?  Surely a passing physics pro has an answer with a better explanation?  Do you weigh less when the moon and/or sun is directly overhead?
 

another_someone

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Re: Weighing Water While under Water
« Reply #5 on: 25/04/2007 20:24:41 »
So this begs the question- do changes in air pressure affect your weight as measured by the scale?  I would think not- maybe this has something to do with air/water pressure acting in all directions while your finite (some are more finite than others) body only exerts downward pressure?  Surely a passing physics pro has an answer with a better explanation?  Do you weigh less when the moon and/or sun is directly overhead?

I had wandered this, but my guess is that air pressure itself should not effect your weight, but air density should (i.e. more humid air, or air that is warmer, should both reduce the density of the air you stand in, and so have the effect of increasing your weight).
 

Offline daveshorts

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Re: Weighing Water While under Water
« Reply #6 on: 25/04/2007 23:30:24 »
If you put some bathroom scales in a sealed plastic bag and then pumped all the air out of it (and it didn't break the bag), you would be measuring the weight of air on top of the scales. A good vacuum cleaner will suck out about 1/5 of the air from a bag...

Similarly if you just put the scales in a sealed plastic bag and put them under water you would weigh the water above them - possibly not to be advised with electronic scales.If you put the scales in a sealed plastic bag and then pumped all the air out of it (and it didn't break th

This is also the reason why things (like submarines and sunken ships) get stuck to mud underwater. If the mud makes a seal so that no water can get underneath then when you try and lift it you are trying to lift the whole weight of the water above it.
 

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Re: Weighing Water While under Water
« Reply #7 on: 26/04/2007 20:04:59 »
Every day at work we check the analytical balances by weighing the same object; a 100g weight.
The measured weight varies a bit; at last part of the variation is due to changes in air pressure.
In theory if you are weighing things accurately you should correct the weight for the buoyancy of the air (people often don't bother). For most things it's about 1 part in a thousand http://www.npl.co.uk/mass/faqs/buoyancy.html#applybuoyancycorr
 That correction depends on the air density and that, in turn, depends on the pressure. Variations in air pressure give rise to an apparent change in weight of something like 1 in 100000. Hardly worth worrying about with bathroom scales (and not often worth worrying about for anything else).
 

Offline that mad man

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Re: Weighing Water While under Water
« Reply #8 on: 26/04/2007 20:18:21 »
This has reminded me of a Cartesian Diver which is odd, as I have just been making one for the kitchen science thread. Not sure if its exactly the same principle though!

Bee
 

Offline Karen W.

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Re: Weighing Water While under Water
« Reply #9 on: 01/05/2007 03:08:38 »
That was what i said ,and her reply was "but if i dropped your bathroom scales in the water their is nothing for the water to push upon " In other words the water cant get underneath the standing surface to push it up.

Ok scales have holes but what if it were sealed.


Dont you just hate it when they sit there think for a minute and complicate things.

This is indeed why I love teaching Kids...LOL LOL..OHHHHH so cool!! Gottcha!!!
 

Offline eric l

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Re: Weighing Water While under Water
« Reply #10 on: 01/05/2007 09:51:35 »
Density of air at sea level is in the order of 1 g/l; density of water is about 1000 g/l or 1 kg/l.  To keep things easy, let us accept that the density of the human body is also 1 kg/l.  If I weigh 75 kg, that would make my volume 75 l.  The push up due to the volume of air you displace would be 75 g. 
Suppose you find a way to double air pressure, and by that also air density, that would give you an extra push upward of 75 g.  If I would climb high enough to reduce air pressure by half, that would substract half of the push up, and my apparent weight (as read on the scales) would be 37.5 g higher.
Conclusion :  if you want to go on top of mount Everest to loose weight, you have to climb yourself (rather than use a helicopter).  Weight loss will be due to the effort of climbing, not to change in air pressure.
 

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Re: Weighing Water While under Water
« Reply #10 on: 01/05/2007 09:51:35 »

 

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