Why Energy Pie Charts?

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Note: This is my first year teaching freshmen physics and it’s also my first year teaching physics using modeling.  Lots to learn!  I’ve had to remind myself of this a lot:

Photo Mar 25, 7 23 27 PMAnyways, this post is about energy.  On with it then:

ENERGY UNIT

So after discussing the various forms of energy and then giving the students some time to play with Phet’s energy skate park (with some guiding questions), we jumped right in to a lab.  The groups were each given an inclined plane and a cart.  We discussed what we could measure, and decided on height of the cart as the independent variable and velocity as the dependent variable.  The velocity should increase as the height increased right?  Let’s see!

Problem: Why are we interested in this relationship?

They didn’t have a clue, so the lab was a total disaster.  Some groups had issues collecting good data (that’s another problem for another blog post) and the rest had no idea why they had collected good data.  I think the reason is because they didn’t start the lab with a good question in mind.  What is the velocity vs height graph supposed to tell us about energy anyways?

Back to the Drawing Board

As I told my students today, it’s just not good science to run an experiment with no purpose in mind.  So where should we start then?  As always, the blogosphere has the answer.  I found this post by Kelly O’Shea.  Starting with pie charts? Sounds good to me.  Maybe that will help us gain a better understanding of energy transfers and then we can head back to the lab.

When I discussed this with my colleague who teaches the same class, he asked why I decided to go with pie charts at all (instead of going with bar graphs).  This comment from Kelly’s blog post shows others have had the same question:

comment 1

You can check the comments on Kelly’s blog post for her answer.  But my answer was that I didn’t really know where else to start and Kelly’s lesson seemed to make sense.  But after doing the activity, I feel like I’ve got a much better answer.  First, here’s what we decided on for our three pies of a ball falling from a really tall building (I used Empire State):

energy pies 1

The discussion that came up based on these pies centered around “what should the proportion of kinetic to potential energy be in the second pie?”.   That was a question that everyone seemed to want answered and no one had a solid answer to.  “Well,” I asked, “what would we need to know in order to determine that proportion?”

BAM!!

energy pies 2

They asked those questions right away with exactly zero prompting from me.  All I did was write them down.  Then, it was super simple for them to see that right there on the board staring them in the face were the exact questions we were attempting to answer with our inclined plane set ups.  Now we have a purpose for our experiment!! We are trying to answer the question, “In what proportion are the gravitational and kinetic energies of the ball when it’s about halfway down?” (or perhaps anywhere on the way down? :).

Other Good Arguments That Came Up

bunny board 2In this whiteboard, you get a visual representation of the students arguing about two things.  First, should there be thermal energy in the first pie?  Someone had drawn it in and then someone else in the group erased it (after a board meeting with another group).  The first student stood by his original pie though.  His reasoning was sound, he explained that thermal energy is internal energy and that the bunny must have some internal energy to begin with because it’s got a certain temperature.  Then argument two is in the third pie, where once again students dueled over whether a wind up toy would be left with some elastic potential energy even after it stopped moving.  It seems the side of “nope” won out on that one.  This next group agreed on the third pie, but not the first:

bunny boardSorry, purple expo marker! You will probably not last long in the hands of this group.

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