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Re: Why does a less dense object rise in a dense fluid surrounding?

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benj <benj.fake@gmail.com> wrote:
> On 1/10/2019 8:00 AM, Odd Bodkin wrote:
>> Arindam Banerjee <banerjeeadda1234@gmail.com> wrote:
>>> On Earth. Why does a less dense object rise up in a dense fluid surrounding medium?
>> 
>> Because the weight of the object is less than the buoyant force of the
>> fluid, which in turn is due the differential in pressure with depth of the
>> surrounding fluid. High school physics.
> 
> Babbling jargon. Try explaining something in English for a change if you 
> can understand things well enough. You are one of those WEstern Idiot 
> dogs that Banjoji is always talking about.

I’m a little surprised you find this jargon. Let’s suppose that everyone
here knows what pressure is and what a fluid is. It is common experience
that pressure increases linearly with the depth in a fluid, in cases where
the density of the fluid is approximately constant. Anyone that has
snorkeled knows this. The pressure on any patch is in fact the weight of
the fluid sitting on top of that patch. 

To make this visually clearer, consider first a pool of water, and imagine
an upright cylindrical volume of this water. If it helps, wrap this volume
in a thin cylindrical can made of plastic of the same density as the water,
so that it has no effect on the following. There is pressure inwards on the
outside of this cylinder due to water pressure. From symmetry, the force on
the left side of the can balances the force on the right side of the can,
at any given depth. But the pressure increases with depth, so the pressure
in the flat top surface of the can is different than the pressure on the
flat bottom surface of the can. The net force exerted by the fluid outside
the can is just then the difference in the forces on top and bottom of the
can. This is called the buoyant force of the fluid on the can. Since the
pressure on the bottom of the can is greater than in the top, the buoyant
force is upward. 

This volume of water (with or without the can) is obviously in equilibrium.
It moves neither up nor down. Thus the downward force of gravity on that
volume of water (namely its weight) is therefore balanced by the upward
buoyant force. This is the statement of Archimedes’ Principle, that the
buoyant force is equal to the weight of the fluid displaced. 

Quantitatively, the weight of the water inside the can is the mass times
the acceleration of gravity. The mass of the water is the density times the
volume. If our van has ends of area A and the top of the can is at depth d1
and the bottom is at depth d2, then the volume of the can is A * (d2-d1),
and thus the weight of the water displaced is (density)*A*(d2-d1)*g. We
just said this is equal to the buoyant force (Archimedes’ principle), and
we said earlier that this was due to the difference in water pressure on
the top and bottom of the can. This is obviously satisfied if the pressure
(force per area) on the top is (density)*d1*g and the pressure on the
bottom is (density)*d2*g. This is the source of our earlier statement that
the pressure increases linearly with the depth in the fluid. 

As the last step in thinking this through, replace the contents of the can
with some substance other than water. The pressure from the fluid OUTSIDE
the can doesn’t change — it doesn’t care what’s inside the can. So the
buoyant force is still the same, equal to the weight of the water displaced
by the can. But the weight of the contents of the can has changed. If the
can is filled with styrofoam, the weight of the contents is now smaller,
and the buoyant force is relatively bigger, and so there is a net force
upward and the can filled with styrofoam rises. If the can is filled with
birdshot, the weight of the can contents is now larger than the buoyant
force, and thus can will sink. 

This is high school physics. 

> 
>>> So exactly why does a cork bounce up when submerged in water?
>>> If the top of a hot air balloon was pointy instead of flat would it rise?
>>> 
>> 
>> 
>> 
> 
> 



-- 
Odd Bodkin — Maker of fine toys, tools, tables

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Thread

Why does a less dense object rise in a dense fluid surrounding? Arindam Banerjee <banerjeeadda1234@gmail.com> - 2019-01-09 21:12 -0800
  Re: Why does a less dense object rise in a dense fluid surrounding? Sylvia Else <sylvia@email.invalid> - 2019-01-10 23:20 +1100
    Re: Why does a less dense object rise in a dense fluid surrounding? Arindam Banerjee <banerjeeadda1234@gmail.com> - 2019-01-10 05:10 -0800
      Re: Why does a less dense object rise in a dense fluid surrounding? Odd Bodkin <bodkinodd@gmail.com> - 2019-01-10 13:32 +0000
        Re: Why does a less dense object rise in a dense fluid surrounding? benj <benj.fake@gmail.com> - 2019-01-10 13:13 -0500
      Re: Why does a less dense object rise in a dense fluid surrounding? Sylvia Else <sylvia@email.invalid> - 2019-01-13 16:31 +1100
    Re: Why does a less dense object rise in a dense fluid surrounding? poraty6@gmail.com - 2019-01-10 16:11 -0800
  Re: Why does a less dense object rise in a dense fluid surrounding? Odd Bodkin <bodkinodd@gmail.com> - 2019-01-10 13:00 +0000
    Re: Why does a less dense object rise in a dense fluid surrounding? Arindam Banerjee <banerjeeadda1234@gmail.com> - 2019-01-10 05:04 -0800
      Re: Why does a less dense object rise in a dense fluid surrounding? benj <benj.fake@gmail.com> - 2019-01-10 13:12 -0500
    Re: Why does a less dense object rise in a dense fluid surrounding? benj <benj.fake@gmail.com> - 2019-01-10 13:11 -0500
      Re: Why does a less dense object rise in a dense fluid surrounding? Odd Bodkin <bodkinodd@gmail.com> - 2019-01-10 19:14 +0000
        Re: Why does a less dense object rise in a dense fluid surrounding? "reber G=emc^2" <herbertglazier0@gmail.com> - 2019-01-10 12:28 -0800
          Re: Why does a less dense object rise in a dense fluid surrounding? benj <benj.fake@gmail.com> - 2019-01-10 19:36 -0500
            Re: Why does a less dense object rise in a dense fluid surrounding? "reber G=emc^2" <herbertglazier0@gmail.com> - 2019-01-11 09:40 -0800
        Re: Why does a less dense object rise in a dense fluid surrounding? mitchrae3323@gmail.com - 2019-01-10 13:21 -0800
        Re: Why does a less dense object rise in a dense fluid surrounding? benj <benj.fake@gmail.com> - 2019-01-10 19:32 -0500
          Re: Why does a less dense object rise in a dense fluid surrounding? Odd Bodkin <bodkinodd@gmail.com> - 2019-01-11 16:20 +0000
            Re: Why does a less dense object rise in a dense fluid surrounding? poraty6@gmail.com - 2019-01-12 23:56 -0800
              Re: Why does a less dense object rise in a dense fluid surrounding? Odd Bodkin <bodkinodd@gmail.com> - 2019-01-13 12:40 +0000
                Re: Why does a less dense object rise in a dense fluid surrounding? poraty6@gmail.com - 2019-01-13 16:01 -0800
    Re: Why does a less dense object rise in a dense fluid surrounding? poraty6@gmail.com - 2019-01-10 16:13 -0800
  Re: Why does a less dense object rise in a dense fluid surrounding? mitchrae3323@gmail.com - 2019-01-10 10:46 -0800
    Re: Why does a less dense object rise in a dense fluid surrounding? Sergio <invalid@invalid.com> - 2019-01-13 15:40 -0600

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