Re: Pb

["John S. Denker" <jsd@MONMOUTH.COM>]

At 11:30 AM 3/9/01 -0500, Scott Goelzer wrote:
> I drop a steel ball onto a one of
> the temperature sensitive color changing post cards (the Exploratorium
> sell these). The card must be on a hard floor.  A color change occurs at
> the point of impact. Pressing the ball against the card and  floor with
> great force and effort causes no change in color. Dropping the ball on a
> piece of wood with the card in between also causes no change in color.
> Demo leads to great student discussion regarding where energy is being
> transferred.


That's a tricky demo, and requires a very special discussion lest the
students draw diametrically-wrong inferences from it.

First of all, there's no reason to believe the card measures energy "being
transferred".  The card measures temperature, which is something quite
different.

Secondly, I suspect the measurement causes a pretty big perturbation in the
collision phenomenon being measured.  I'll bet the steel ball hitting a
hard surface bounces markedly higher without the card than with the card.

In this situation, we have neither the ideal inelastic collision, nor the
ideal elastic collision.  That's fine;  it's good to look at non-ideal
situations.  My concern is that the card measures only one of the
nonidealities, while other small effects (and indeed big effects) go
unmeasured.

If the objective is to demonstrate energy transfer, aren't there better
ways to demonstrate that?  (Carts on an air track come to mind.)

If the objective is to demonstrate energy dissipation, aren't there better
ways to demonstrate that?  (A nice steady rubbing motion comes to mind.)