Re: buoyancy puzzle (long!)

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

Maybe I'm missing a subtle point, but the buoyancy force it due to the
difference in pressure between the top and bottom of the object.  With the
two blocks glued, they become one object of weight W.  The buoyancy force
will be -W.  The net force is zero and would be at any depth at which you
might place this.  I can't see where the two different densities (top and
bottom) matter, once the blocks have been mated.  So I say they stay
together, neutral buoyancy, no matter what.

Rick


----- Original Message -----
From: "Carl E. Mungan" <mungan@USNA.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Wednesday, May 01, 2002 12:53 PM
Subject: Re: buoyancy puzzle (long!)


> John Denker wrote:
>
> > In this case, imagine that the glue is a form of rubber cement.
> > Sticky, but somewhat compressible.  When you subject the glued
> > blocks to great pressure, the glue compresses a bit (decreases
> > in volume) and exerts a pressure on the glued faces.
>
> Okay, fine. But now back to the original question. Here are two
situations:
> 1. I hold a block of wood (density = 0.8 g/mL) underwater against the
> buoyant force. A small distance beneath it, I hold a block of metal
> (density = 1.2 g/mL) in my other hand. I let go. The wood rises and
> the metal sinks.
>
> 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?
> --
> Carl E. Mungan, Asst. Prof. of Physics  410-293-6680 (O) -3729 (F)
>        U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
> mungan@usna.edu    http://physics.usna.edu/physics/faculty/mungan/