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Conservation of Energy Laboratory Activity
Name: Eugene
Status: educator
Grade: N/A
Location: MP
Country: N/A
Date: 7/12/2005
Question:
Hi Folks
I am teaching Physics course in summer school. I have checked libraries
and web sites for a good "The Law of Conservation of Energy" lesson plan.
Nothing interesting, only activities on the motion of a pendulum bob and a
roller coaster. May be you know what else I can use to grab students
attention?
Replies:
I am not sure this is what you want, but it certainly should "grab students
attention". The conservation laws are more than they are "cracked up to
be". They are neither simple nor straightforward as most of us were
"taught" (and I use the word loosely). Emmy Noether, a female in a man's
world of the early 20th century, (which should be a plus for your female
students) proved that for every law of conservation there exists a
mathematical symmetry in the dynamical mechanical laws. For example, the
conservation of energy (taken as "given" to most of us) is true if and
only if there is a symmetry of the laws of motion with respect to time
(that is, the laws work just as well for t > 0 and t < 0)!). And the
conservation of momentum is true if and only if the dynamical laws of
motion work just as well for (x,y,z) > 0 and (x,y,z) < 0. The following
sites will put some mystery back into the shell game that was foisted upon
most of us regarding the conservation laws. The math may be advanced but
that is OK too. There is nothing wrong with suggesting to students that
things are more complicated than they appear. I recall a quote attributed
to Richard Feynman that has stuck with me: "If someone tells you he/she
understands quantum mechanics, they do not understand the problem." The
same applies to the laws of conservation of dynamical variables.
http://www.answers.com/topic/conservation-law
http://www.upscale.utoronto.ca/GeneralInterest/DBailey/SubAtomic/Lectures/L
ectF13/Lect13.htm
http://www.nationmaster.com/encyclopedia/Conservation-law
Vince Calder
Dear Eugene,
My favorite is a large pendulum with a bowling ball or similar for the bob
and a 20 foot or longer rope. I would climb on a ladder and place the back
of my head against the wall, hold the bowling ball against my nose and
release it. The ball would then swoop across the lecture hall and return to
slightly depress my nose. Meanwhile, I would (nervously) talk about my
faith in conversation of energy. Often at the first swing I would step
aside and catch the ball just before it arrived at my nose in an attempt to
build suspense while emphasizing what the ball could do to my nose..
A little hokey, but it always seemed to go over rather well. I also played
with a super ball and talked about gravitational potential energy
transferring into kinetic energy, then elastic potential energy, and back
again. You are right, though; it is hard to think of really interesting
demonstrations on conservation of energy which are also good physics.
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
Eugene,
I have often found that collisions can draw attention. Unfortunately, a
collision that clearly conserves energy (i.e. elastic) can be difficult to
create.
If your school has devices known as collision carts, or if you can obtain
carts that have very little friction, you may be able to work it out.
Magnets mounted on the carts can provide a collision where the carts never
touch one another. This eliminates loss of energy to sound. Very little
friction can minimize loss of energy to heat.Kinetic energy before the
collision should be only a little bit greater than kinetic energy
afterwards. This is due to air resistance and a little friction.
Another option involves pendulum magnets. For a circular magnet, you need
two strings to hold each magnet. A bar magnet will require four lines, two
at each end. There can be no "side-to-side" motion. Hang the magnets
carefully. If the magnets had no charge, they would hang such that they
just touched. With the magnetic charge, they will repel each other. Use
long strings to make height measurements easier and slow down the
experiment. The magnets must be strong enough to prevent contact during the
experiment. Measure initial and final heights. Initial height gives
initial energy. Final height in each magnet gives final energy in each
magnet. The magnets do not need the same masses, but the mass of each must
be known. Taping something small and heavy to a magnet to change masses can
be interesting. Larger mass reduces the effects of air resistance but does
NOT speed up the experiment.
Kenneth E. Mellendorf
Physics Instructor
Illinois Central College
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