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"John S. Denker" <jsd@MONMOUTH.COM> 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.)
Doesn't the impact damage the micro-encapsulated liquid crystal?
Are you
sure you are looking at a temperature change and not a physical damage
artifact?
I suspect that this interesting demonstration may possibly be
sailing under false colors. I understand that temperature
indicating cards feature a thin film of temperature sensitive
liquid crystal. If you apply enough pressure, I fear that the
liquid will be squeezed out with a conseqential color change
no matter what the temperature.
As a check, using a press to slowly apply pressure from a cooled
ball onto such a card might demonstrate some interesting color
change also.