Re: [Phys-L] The Make-Believe World of Real-World Physics

[brian whatcott <betwys1@sbcglobal.net>]

On 07/17/2013 07:25 PM, Anthony Lapinski wrote:
> > > For the helium balloon problem, I first do a related demo in class. It's a pulley, cart, and hanging mass, and I attach a horizontal level to the cart. When the mass is released, the cart accelerates to the right. Which way does the bubble move? This is about inertia.
On Jul 17, 2013, at 11:30 PM, John Denker responded:
> > OK, that's one experiment.  Here's another:  Just now I grabbed a brownie pan, approximately 40 x 30 x 5 cm.  The details don't matter, over a wide range. I put about 2 cm of water in the bottom.
> > I grabbed a slab of hardwood, approximately 30 x 24 x 2 cm, and
> > floated it in the water, initially centered over the middle of
> > the pan.  Then I jerked the pan hard to the right.  Now, relative to the pan, which way did the wood move?

I am not comfortable that faced with an instructional approach from 
Anthony L.  intended to help student insight, John D. responded with a 
counter-example likely to confound student (and probably instructor) 
expectations. This reminds me of trial lawyer dialog rather than physics 
instruction dialog.

Rather than drifting away from two media behavior in a shifting gravity 
vector by citing a three media experiment in a shifting gravity vector, 
it would have been more relevant to note the behavior of a spherical low 
density wood ball in a transparent box COMPLETELY filled with water. 
That, I suggest, has some comparability to the spirit level and helium 
balloon models

Brian Whatcott    Altus OK