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Re: buoyancy puzzle (long!)



"Carl E. Mungan" wrote:

2. Same as above, but this time I completely replace the water in the
gap between the two blocks with rubber cement. I again let go. What
happens? Under what circumstances (if any) would the two break apart?

Let's consider the case where the glue goes in as a liquid and
then hardens to a slightly squishy solid.

a) While it is liquid, it comes into hydrostatic equilibrium
with the water it is replacing.

b) You continue to hold on while the glue hardens. During
this time, you don't notice any difference in the force
equation. Water at pressure P behaves just like liquid
glue at pressure P.

c) As the glue hardens, if you absolutely don't move your hands,
you won't notice any change in the force. If you do move
your hands, after the glue has hardened you will discover that
the spring constant dF/dx increased radically. In fact, an
imperceptible dx suffices to create enough force to overcome
the difference in weight and hold the blocks together.

To repeat: It would be completely unphysical to think of
the insertion of the glue as causing an important change in
the pressure or force. The hardening of the glue causes
a radical change in dF/dx, which is something else entirely.