Re: [Phys-l] Definition of upthrust or buoyancy

[brian whatcott <betwys1@sbcglobal.net>]

 About tires....
I have to say, that this very topic crossed my mind too.

I could make no headway at the time with a certain well-loved professor 
of Physics and Chemistry
when arguing for  the concept that  upper surfaces of an inflatable (a 
tire in that case)
could carry an appreciable axle weight.

The issue seems to be that tension structures such as the usual 
inflatable, can carry different
forces in  orthogonal surface axes as well as react forces normal to 
their surface.
This can happen by a change in the preexisting tension .
  Here (under water)  we could have a tension structure with balanced 
internal
and external pressures acting to compress the envelope, independent of 
the tension that may
or may not exist. A balloon inflated only to its "rest volume" sees no 
skin tension for nominal
 top to bottom depths, for example.

To conclude, it is amusingly potent gotcha material, to invoke a surface 
under tension in one surface axis,
in greater tension in an orthogonal surface axis, and in compression in 
a third (normal) axis.

Brian W

On 10/21/2010 10:29 AM, Philip Keller wrote:
> Two thoughts...
>
> 1. This reminds me of an older thread about what holds up a car or bicycle.  Consider the spokes of a bicycle wheel:  they are all in tension.  It is the difference between the tension in the upper and lower spokes that balances the weight of the bike.  Similarly, it is the difference in pressure on the upper and lower surfaces that produces the buoyancy force. Same for the upward force that you apply with a suction cup.  After all, the inside of the cup does not have to be a complete vacuum for the cup to work.  But if there is air inside the cup, it exerts downward pressure.  But less downward pressure than the upward pressure coming from the atmosphere.
>
> 2.  The argument that "if you cut off the piling it floats so it must be in tension" had me convinced for a while.  Or the reverse argument that if you wanted to add another layer to your piling, you would need glue.  But you wouldn't need glue -- you would just need to have smooth enough surfaces to push out all the water (which brings us back to the question that started the thread).  It is not a realistic way to make a piling, but it does show that without any upward component to the water pressure, the piling is in fact in compression.  Cutting part of it off introduces another surface for water pressure to act on.