Re: [Phys-L] [SPAM] Re: Physics, Errors and Different Teaching Styles

["Anthony Lapinski" <Anthony_Lapinski@pds.org>]

Great discussion! Archimedes' Principle is the most challenging topic in
fluids. Even my honors students struggle with it. I could do an entire
test on it, but I don't. You really need many in class demos, questions,
and peer instruction to help students understand these ideas.


Phys-L@Phys-L.org writes:
> > On 6/26/2012 6:15 PM, Jeffrey Schnick wrote:
> > > > #3. "A boat carrying a large rock Is floating on a lake.
> > The boulder
> > > > is thrown overboard and sinks. The water level in the lake (with
> > > > respect to the
> > > > shore)
> > > >
> > > > 1) rises
> > > >
> > > >  2) drops
> > > >
> > > >  3) remains same"
>
>
> My favorite solution to this old puzzle said it this way:  while on the
> boat, the boulder displaces an amount of water equal to it in weight. 
> Thrown overboard, the boulder now displaces water equal to it in volume. 
> Since the boulder is denser than water, we know that the first volume is
> bigger.  So it displaces less water submerged than when it was in the
> boat.
>
> One advantage of this solution is that it doesn't depend on the normal
> force from the lake bottom.  After all, the water level drops even before
> the boulder reaches the bottom.
>
> Also, this solution method handles a common variation on the problem: 
> throwing wooden blocks  overboard instead.  Now, they continue to
> displace a volume equal to their weight, so no change in water level.
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