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Re: Pb



From: "John S. Denker"


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!


But the ball stops bouncing eventually. Each bounce is lower than the previous. I
am trying to show where (some) of that "missing" energy is going.

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.


A ball is dropped, bounces, and comes to rest. Where did the GPE go? Can I
provide any evidence at all besides _telling_ them it changed into thermal energy.

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.


I disagree. Total changes to the ball and floor temperatures are small, but at the
moment of impact, only a small portion of the ball is in direct contact.

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.

------------------------------


But never the less the card changes color. I am not claiming all of the energy
goes to changing the color of the card, but the collision between two surfaces
does cause a temperature change right at the impact point. I cannot wait to set
up the demo described in the previous post of burning paper between two colliding
metal spheres.
--

*****************************
S.Goelzer
Physics Teacher
Coe - Brown Northwood Academy
Northwood NH 03261
603-942-5531 ext43
sgoelzer@coebrownacademy.com
*****************************