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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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