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Re: Pb



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

At 03:35 PM 3/12/01 -0500, S.Goelzer wrote:

Whoops, I forgot about the sacred heat cow on this list.

That was absolutely not the point I was making. I systematically didn't
use the word "heat" precisely so we could sidestep this quagmire.

We tell students that GPE changed to KE during the fall, but I wish to
show that after the fall there is still evidence of energy. The energy
"goes" (verb of your choice) to another form. Temperature is _evidence_ of
the increase in thermal energy not the energy itself. Footprints are not feet

I'm totally lost here. When I drop a steel ball on a sufficiently-hard
surface, I get a tremendously efficient bounce. The "evidence of energy"
is not hard to see: it's in the still-moving ball!

> 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.

Yes, but not much.

... which is further evidence that we're having two separate
conversations. We can't possibly be talking about the same things.

What happens if you make a stack of three or four cards, and bounce against
that?

> 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.
>

I wish to show that the energy did not just disappear. This demo provides
_simple_ evidence that a change did occur and something is still moving
even if it cannot be seen or felt directly.

But the ball flying away can be seen and felt!

The ball transferred 200% of its initial momentum during the collision, but
there's no reason to believe it lost more than a small percentage of its
energy.

I'm just totally bewildered.

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

How do I get a tangible result? From a student's point of view, a pasco
cart loses speed after each bounce, but nothing is noticeably warmer.

The points being
-- In this case not much energy is lost in the collision.
-- When the ball hits the hard floor not much energy is lost.
-- Even if the collision were completely inelastic the resulting rise in
temperature would be fairly small, because heat capacities tend to be
large, thermal energies tend to be large, and kinetic energies tend to be
small, on laboratory scales. Also a goodly part of the lost energy winds
up in the air, which is inconvenient to measure.

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

OK, but I still need to convince HS students of where the GPE ends up.

The vast majority of it winds up in the ball as it flies away. If you've
proved it winds up in the card, or transferred through the card into the
floor, you've proved the wrong thing!

If the floor really is hard, and the card really is non-perturbative, then
air resistance is the dominant dissipation mechanism, and the card sure
doesn't measure that.