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13 December 2014

AP in-class laboratory exercise: Energy (And more on different approaches in 9th and 12th grade)


Above is an example of an in-class lab exercise for AP-level seniors
When I introduce a new topic in 9th grade conceptual physics, I hand out a sheet with a few facts and equations, then I dive directly into guided laboratory exercises.  You can see one set of such exercises, about collisions, here


I don't do any discussion, or example problems, or anything at all with me talking to the class. There's no point -- the freshmen don't have the attention span to listen, and they don't have the abstraction skills to apply what I show them to future problems.  Therefore, the 9th grade in-class laboratory exercises walk the students step-by-step through the solution to a problem, then guide them through the experimental verification of their solution.  No one can tune me out, because I'm not talking. Instead, each student himself has to wrestle with the problem, showing me his answer to each step. When someone does a step incorrectly, I help him, and send him back to his seat to try again.

When I tried the same approach with AP-level seniors this year, it didn't work.  

A freshman who's told his answer is wrong generally looks sheepish, goes back to his desk, does the problem right, and finally looks happy as a mollusk to move on.  

A senior takes the wrongness of his answer personally.  While the freshman just accepts my word that his answer was wrong, the senior tends to make ever-more-ridiculous arguments at me to justify his incorrect reasoning.  Seniors aren't sheepish about wrong answers; no, they're defiant, as if it were my fault that the universe doesn't work the way they want it to.  

On the other hand, I've had good success over the years holding seniors' attention with quantitative demonstration lectures.  So after Thanksgiving break, I went back to my previous approach in teaching the work-energy theorem.  It went well... I raced through a bunch of energy problems at the board over just a few days.

Then, after those few days of me solving problems and showing demonstrations, after a few days of problem solving on each night's homework, I handed out this in-class lab exercise.  

Each student got a different sheet.  The picture above shows problem 1 -- but the link includes seven different sheets, with seven different energy problems.  Three involve carts on a track, three involve objects on vertical springs, and one involves a sliding block.  Each problem requires students to solve in variables, then use semi-quantitative reasoning to produce a prediction.  The experimental verification can be done with motion detectors and/or photogates -- no other equipment required.

The seniors did much, much better this time.  They were no longer hostile -- they felt like I had shown them how to solve the problems, so that if they got something wrong, it was their own dang fault.  

And that was interesting... the freshmen never worried about blaming themselves or me for a wrong answer -- it was just wrong.  The seniors got very snarky if they felt that I hadn't showed them the correct approach at the board, or if I hadn't mentioned all relevant background information out loud in class.  They pouted at their seat if they were turned back more than once to try again.  

But once I had done my duty lecturing at the front of the room, the seniors enthusiastically took to the same kind of open-ended independent lab exercises at which they had thumbed their noses earlier in the year.

I will likely come to some broader conclusions about seniors in the new AP course after I experiment a bit more with my class this year.  I'd love to hear other teachers' experience with these or similar in-class exercises.  

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