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Lesson Goals and New Content
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The students will have their first
encounter with the idea of energy conservation and total mechanical
energy. The goal is for them to experience how energy doesn't
disappear or appear, but rather is converted from one form to the other.
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Procedure
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Opener (~10 min.)
One way to introduce the exercise to the students is to first do
a little exercise having them identify the types of energy of a ball at
different stages. Throw the ball into the air, and draw a diagram
on the board. Then ask the students to identify the types of
energy of the ball as it leaves your hand, as it reaches the peak of its
motion, and somewhere in between. (Leave your diagram on the
board.) Leave the question open and unanswered for now; it is the
driving question behind the lesson. Once you have formulated the
question with them, transition to the worksheet.
Development (~25 min.)
Release the students into groups of 3-5 and let them work through
the exercise. They will start by dropping a ball and measuring
the height to which it bounces, which should only take a few
minutes. After that, let them work through the questions in the
groups, while you walk around and help them out. In my experience,
most groups should be able to make it through questions a-f in the
allotted time, but you can extend it as needed and let the closure carry
over to the next day. Part g on the sheet will frustrate some of
them, but is a lead into a different lesson, so don't give away the
answer at this point. (If it frustrates them too much, make that
question optional and return to it later.)
Closure (~10 min.)
Briefly go over the worksheet with the class. You should
highlight that the initial potential energy of the ball has turned into
kinetic energy as it hits the ground. Use the calculated potential
and kinetic energies from one group and fill them in on the diagram from
your opener. Then, make up a number in between the two and write
that as the potential energy at your middle point. Have the class
guess what the kinetic energy would be. Most of them will probably
see the pattern and get the right answer, perhaps with a little
guidance.
Only now will you introduce the term energy conservation and total
mechanical energy. It is my experience that textbooks have many
elaborate equations stating energy conservation, explicitly writing out
all potential and kinetic energy terms. Students seem to think of
these equations as separate and having nothing to do with each other,
which confuses them and makes energy conservation a very abstract
concept. I find it best instead to emphasize that ME=PE+KE,
and that MEinitial=MEfinal. This
contains the central idea you want to embed in their minds, and they
relate better to this, having just seen it themselves in the activity.
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Evaluation
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In addition to the continuous evaluation
during the lesson, I would suggest following
up with a set of practice problems covering the basic idea of energy
conservation, for example as homework for the following day. The
concept of energy will also come up many times later in a physics
curriculum, presenting many opportunities to continuously evaluate the
students' understanding.
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Extensions
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Part g on the worksheet is more advanced,
and depending on how quickly your class progresses through the different
parts, you might want to use it as an opener for a different
discussion. It can be used as an opener to a discussion on heat as
a form of energy.
Part g contains a discrepant event: the ball started with some amount
of energy, and we just concluded that energy is conserved; yet it ends
up on the ground not moving (no potential or kinetic energy). To
lead on the discussion, ask what happens when you run down the stairs
running your hand against the railing. The students should realize
that friction generates heat. Now ask why a car engine has to keep
working, even when the car drives at a constant speed. The
students should realize it is because of friction, which means the
engine's work generates heat.
When I did this lesson, we had already gone
over work with the class. A lesson linking work and energy
together follows naturally after this one, focusing on the central
concept that you can add energy to an object by doing work on it.
I like doing this by having them calculate the energy added to a
book being lifted a height h at constant speed, and then
comparing that to the equation for potential energy. It builds
well off this lesson, as the students need to understand energy
conservation to appreciate the relationship between work and energy.
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