Re: [Phys-l] A Crude Attempt at Analysis

[John Mallinckrodt <ajm@csupomona.edu>]

Chuck Britton wrote:

> At 1:12 PM -0700 11/5/10, John Mallinckrodt wrote:
>
> > Just a thought that might help.  Imagine that you place the physical 
> > cube JUST in contact with the bottom so that all fluid is excluded, 
> > but the bottom exerts NO significant force on the cube.  Hopefully 
> > you see that the cube will be DRIVEN downward into MORE significant 
> > contact with the bottom.   It is the resulting compression of the 
> > bottom that results in it exerting a greater--and ultimately 
> > balancing--upward force.  The same mechanism explains how horizontal 
> > surfaces always manage to exert JUST the right amount of upward 
> > force on whatever sits on them.
>
> ok - when the physical cube is just a tad off the bottom - we agree 
> that the mathematical summation of forces over all six sides must be 
> identical to the Archemedian Lift force. This summation really 
> doesn't care what's inside the defining surface. When does the 
> summation begin to care what's inside the surface.

Okay. One more try. 

The summation of forces BEGINS to care about the contents (specifically the mass) of the box as soon as you release the box and it starts falling toward the surface because NOW the mass of the box itself helps determine the future course of events.  Eventually, when the box comes back into equilibrium, the surface will be very slightly (VERY slightly!!) compressed by the force being exerted downward upon it.  The MORE massive the box is (i.e., the "heavier" it is), the MORE the surface will have to compress to be able to supply the required upward support force.

Take a look at

    http://alienryderflex.com/equilibrium/

and see if that doesn't help.  John Clement might have better references for this so-called conceptual "bridging analogy."

John Mallinckrodt
Cal Poly Pomona