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28 March 2013

Lab quiz: collisions


The AP Physics development committee got serious about testing laboratory skills nearly two decades ago.  The Regents has included some lab questions recently.  The new AP Physics 1 exam will have all sorts of experimentally-based questions.  We get it -- a physics class includes experimentation, not merely abstract problem solving.  


Thing is, abstract problem solving skills are easy to evaluate -- give the student a problem, see if he can solve it.  Lab skills are incredibly difficult to evaluate.  

A lab practical is the best way to test whether students can function in a lab setting.  A couple of times, when I've had small classes and a 3-hour block for an exam, I've given an actual practical piece of an exam.  Specifically, I've given students a circuit diagram, and said, "hook it up;" I've given them a circuit and told them to measure the current through and voltage across one of the resistors.

From a practical perspective, though, the true lab practical is difficult to implement.  When I've got 50 students for a 2-hour exam block, I can't rotate them all through the lab piece and still have time for the rest of an exam.  And there's no practical way to scale up a lab practical to a national, state, or district-wide exam.  

The practical* reality is that it's important to show students that their lab skills are being evaluated; but just grading sheets they turn in after a lab, or grading lab reports, or giving marks for lab conduct, doesn't cut it.  I think students need to see tests and quizzes with questions posed in a laboratory setting.  

*Okay, okay, not funny anymore, got it. 

You can certainly use AP-style laboratory-based questions.  Some of these go through the data analysis process, which can be seen as a test of abstract problem solving rather than of true experimental ability.  Some ask to describe a procedure for measuring something, which is much closer to an authentic evaluation of experimental skills, but is simultaneously a writing test.  I know I have students who could do the postulated experiment, but who can't explain how to do it.*

* That doesn't mean "describe a procedure" questions are bad.  Writing is part of experimental physics, too.

If you've never seen the AP Physics lab questions, take a look -- each Physics B exam since 1996 has included one lab based question.  Use them.

I've taken to doing shorter, more frequent, less formal quizzes involving laboratory work.  

For example, my freshmen just finished a whole set of experimental collision problems.  They banged carts together, then used a motion detector connected to a Labquest2 to determine the speed of a cart before and after the collision.  After a few days, most got quite good at reading and interpreting the velocity-time graph. But I know some were relying on friends to help them read the graph -- when I would ask these folks "so, where did the collision occur?"  they would look blank, and guess.

So, I sent the following email to the class folder -- without any pictures, of course.  Tomorrow in class I will hand out the indicated questions as a quiz, with a picture similar to the one at the top of this post included.  Point being -- if you really understand how to get cart speeds from the labquest, this quiz will be a piece o' cake.

On Friday, we will take a short daily quiz.  Here are some (not all) of the questions that will appear on the quiz.  There will be a photograph of a labquest associated with these questions.  

Questions 1-5:  In the laboratory, you press play on the labquest.  Then, you push a blue cart toward a stationary red cart.  The blue cart collides with the red cart.  Above is the reading of the motion detector that is located behind the blue cart.

1 How do you determine speed from a velocity-time graph?

2 On the velocity-time graph, circle the graph immediately before the collision.

3 On the velocity-time graph, draw an X on the graph immediately after the collision.

4 What was the blue cart’s speed immediately before the collision?

5 What was the blue cart’s speed immediately after the collision?


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