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

[Philip Keller <PKeller@holmdelschools.org>]

> 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.