Re: [Phys-L] Nice question on buoyancy and balance

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

Good idea! I agree we don't want to stump the students. They already find
physics very challenging.

When I ask (peer instruction) questions on Archimedes Principle, I start
with the basics and build from that:

Drop marble into cylinder of water. What happens to buoyant force as it
sinks?
Two bowling balls in aquarium -- one floats, one sinks. Compare buoyant
forces.
Ship sinks in canal lock. What happens to water level in lock?
Ice cube melts in beaker of water. What happens to water level?
Two beakers with water at same level, but one has cork floating. Compare
scale readings.
Stick finger in beaker of water. What happens to scale reading?

So many more. You could do an entire unit/test on this stuff.


Phys-L@Phys-L.org writes:
> Driving to work this morning, I had another thought about this problem
> (the
> original one that started the thread).  I was thinking about how to
> present
> it to my students.  Since the goal is not to stump them with puzzles, I
> think I would first ask them about the variation where the ping pong ball
> is held in place by the dowel.  I believe that one is easier to
> understand.
> Same amount of water exerting same buoyancy force on both sides of
> balance.
>
> Then I would go back and take another look at the question about the
> window-washer held by a rope which goes over a pulley and comes back down,
> in one case to the window washer and in another case to a hook coming out
> of the building.  The tension in the first case is half that of the
> second.
> Ropes are not magical, free-floating force providers.  There has to be
> something at the other end, also experiencing tension.
>
> Then I would go back to the ping pong ball and replace the dowel with a
> rope tied  to the bottom of the beaker.
>
> Presented in this order, it is not as much of a puzzle, which is fine with
> me.
>
>
> On Wed, Jan 29, 2014 at 9:28 PM, John Clement <clement@hal-pc.org> wrote:
>
> > As I said in the second message, OOPS.  Write in haste repent in
> leisure.
> > Remember "Independence Day".
> > It is always wise to look at all messages and read thoroughly before
> > writing.
> >
> > John M. Clement
> > Houston, TX
> > > Carl specified an UNtethered ping pong ball.
> > > Are you of the same opinion still?
> >
> >
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