<quote>
You wrote that discussing an egg drop vehicle experiment in terms of
energy
is wrong. I heartily disagree. It is very instructive for students to
design ways to not only increase the time of the impact (as you stated)
but
to decrease the required momentum change by decreasing the kinetic
energy of
the vehicle (through drag force or rotation). I don't find physics can
be
compartmentalized so much as you state. Energy and momentum should be
considered together by students, because they "occur" together."
<end quote>
In the rules I have seen for egg drop experiments, drag devices (like
parachutes) are not legal. The egg-carrying device must hit the floor
after a free fall. All entries in the contest must hit the floor at
the same velocity.
Even if this were not so, I would not view that we are trying to reduce
the momentum change by reducing the kinetic energy. I would view that
we are trying to reduce the momentum change by reducing the impact
velocity. Granted, reducing the impact velocity, reducing the required
momentum change, and reducing the impact kinetic energy amount to the
same thing. There just isn't any reason to invoke kinetic energy. If
there's no reason to invoke it, why cloud the issue with extraneous
physics?
This whole experiment can be carried out, and properly designed
egg-vehicles can be built, without ever mentioning energy. This is
true regardless of whether reduced impact velocity is allowed or not
allowed.
I certainly agree that many aspects of our physics equations and
conventions occur simultaneously in most experiments. And it is
sometimes useful to examine everything going on. Obviously it is
worthwhile to bring energy considerations into discussion of collisions
because we find it useful to divide collisions between elastic
collisions and inelastic collision. If we have elastic collisions,
then conservation of kinetic energy gives us additional equations to
analyze the motion (in addition to the momentum equations). But once
we establish the collision is inelastic, then we don't have any
conservation of energy equations that can bring any more insight into
the motion; we only have the momentum equations. (Unless we somehow
can know the percentage of elasticity... something we typically would
not know until after we solve the momentum equations.)
In contrast to considering everything that "occurs together" I would
say that is important to guide students to view experiments with the
goal of trying to ascertain the most critical aspects. We don't want
them to lose sight of the forest because of the trees. Thus, my beef
with bringing KE into the egg drop experiment is that it is not needed,
and worse, it can obfuscate the fact that this is primarily a delta-p =
F(delta-t) experiment. Seat belts, air bags, catching baseballs,
egg-drop experiments are all good examples for teaching
impulse/momentum. They are not very good examples for teaching
conservation of energy.
A science booklet published by the State of Ohio Department of
Education intended to help high-school science teachers actually
recommends the egg-drop experiment as a way to teach conservation of
energy, and it never mentions momentum or impulse. As I read it, it
just doesn't make any sense. The writers of that booklet clearly lost
sight of the forest because of all the trees.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817