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Re: [Phys-l] Teaching energy



Jack,
I'm unsure of what you are asking...so I will describe what I do with energy and hope your question is answered...
1) I start off by pointing a stretched rubber band at a student (frequently one who has a tendency to feel comfortable reacting in front of peers)...the student "flinches." I ask the class, why is the student flinching...they respond because "you could release the rubber band and shoot the student and it would hurt." I ask, "If I pull it back further, does that correspond to increased flinching?" They respond "Yes." Then I point it away from all students and shoot the rubber band. I tell them that pulling back the rubber band further doesn't necessarily have to be associated with increased pain...it could be associated with increased range. I ask the students for a term that they think is associated what I give the rubber band to increase the pain, range, height, etc. and usually, someone offers up energy. Then I ask the class to help define energy based on the demo and usually, "the ability to cause change" is the resulting working definition (no pun intended).
2) Next, I might ask the students "how did the rubber band get the energy?" They will say that I gave it to the rubber band. I will ask, "Where did I get the energy?" They will say from food...and so on and so on. So my focus is where did the energy come from and where does it go...in other words, what/who possesses the energy.
3) I will then produce a spring and stretch it by adding mass and ask them the same series of questions. The students will make the connection to rubber bands immediately. Then I will ask the students to find the relationship between weight added and stretch. Of course, they will find Hooke's Law from stretch vs. weight. Since the students have been evaluating intercepts, slopes, and areas under curves for 6 units now, I will ask them what is happening to the incremental area under the curve as more and more weight is added...they will say that the incremental area is increasing. Then I will return them to the discussion with the rubber band and ask them "what is the spring acquiring when it is being stretched more and more?" and the students will say energy. So out pops an equation for the energy stored in a spring as 1/2kx^2. I just call it energy. How did the spring get the energy...students will say that weight stretched spring...out pops a mechanism to give an object energy...work (defined force times distance). I could give it energy by stretching it or a mass hanging on it could give it energy...the important thing is the force causing the stretch.
4) We then do similar experiments to come up with a way to measure the energy stored in the gravitational field (mgh) and the energy from the motion of an object (1/2mv^2). It is in the discussion of a book on a shelf on the earth or on the moon that we decide that the book itself doesn't possess the energy but rather the configuration of the masses in the gravitational field. As Gregg Swackhamer would say...who decides which mass keeps the energy (apologies if I butcher Gregg's interpretation...I am expressing my interpretation of Gregg's interpretation). According to 3rd Law, the symmetry of the interaction objects means that neither can possess the energy while the other doesn't...hence the idea that the field possesses the energy.
5) Once students have done experiments to come up with a way to calculate the energy stored in a spring, in a gravitational field, and in the motion of an object, we revisit the concept of work as a means for a system (or object) to give up energy or acquire energy. We then include the other ways in which systems (objects) can acquire or give up energy, heating and radiating. It is also here that I play Feynmann's audio lecture of "Dennis' energy blocks." To answer your question, the students express their enjoyment with laughter and knowing nods.

I will exit this discussion now knowing that I am unequipped to debate energy at a level of understanding beyond this...my understanding of Lagrangians and Hamiltonians is a distant memory. Please be kind...I typed this while monitoring a crowded study hall so I assume unintentional errors...but I don't have time to edit.
Regards,
Rob

Jack writes
<Hi Rob-
< I'm surprised. In my experience, unless the students are given
<talks during the showings, they are pretty passive observers. What do
<your students do to show their enjoyment?
< Regards,
< Jack



On Sun, 1 Oct 2006, Spencer, Rob wrote:

w.r.t. Ron McDermott's comments on teaching high school energy...amen!

p.s. Feynmann's Dennis the Menace Lecture on "energy" blocks parallels
the modeler's approach to teaching energy and the students enjoy
the audio lecture from Cal. Tech very much.
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