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14 December 2016

Quizzes to follow up AP Physics 1 problem solving: Try composing tweets.

Old-tymie physics questions would simply ask, "Calculate the horizontal distance block B travels after it leaves the table."  Such a question will be vanishingly rare in AP Physics 1.

Case in point: consider 2010 AP Physics B problem 1 part (d).  You have a block pushed by a compressed spring.  The block collides with another block, then falls off a table.  No analysis, no articulation of principles necessary... just perform the calculation.

Don't get me wrong, 2010 AP Physics B problem 1 is a fantastic question.  It combines in one simple situation the three canonical approaches to classical mechanics: force/kinematics, momentum, and energy.  I assigned this problem verbatim to my AP class last week.

Of course, I encourage collaboration in and out of class, as do most of us.  Thus, a significant fraction of the class got the approach right because someone pointed it out to them.  No, that's not "cheating," that's working together.  Students engaged the problem individually, most got stuck somewhere, and then through conversation and direct advice, they figured out what to do.  Awesome.

I will certainly grade this problem.  Presenting the solution clearly is an important skill to develop.  And by grading the problem, I provide incentive to engage in the collaborative process.  I can tell the difference between Fred, who just kinda blindly followed a friend's work, and Jim, who himself showed each step clearly.  At this point I don't care that Jim showed each step clearly because George told Jim how to do each step.  Jim wrote out his work, and so made progress toward personal understanding.

Nevertheless, I need to evaluate my students' personal understanding of the process.  I need to help my students evaluate for themselves what they understand and what they don't.  After all, the AP exam is not a collaborative exercise.  Everyone, by May, needs to be able to independently figure out how to approach this type of complex problem.

More to the point, my AP Physics 1 students must be able to do more than just perform the calculational procedures that lead to a correct answer.  The exam might ask, "Explain how you would calculate the distance block B travels after it leaves the table."  And the response can't be "I multiply 1/2 times 250 times 0.15 m squared, then plug into p=mv."

So I give a quiz.  What kind of quiz can you give based on this problem, Greg?  I'm glad you asked.

Sure, you can give the same problem and change the numbers.  That's okay.  It doesn't put students on the track toward answering AP Physics 1 verbal response questions, but it's a start.

You could also change the situation slightly... have the block initially slide down a ramp rather than be pushed by a compressed spring.  Or eliminate the collision.  Or put the table on Mars.

I've discussed in this post how I ask for annotated calculations in order to check for understanding.  An interesting quiz might present a full solution in numbers and ask the student to annotate the calculation to explain each step.

Even then, students have a hard time recognizing what parts of a solution are important to annotate.  They want to describe the arithmetic: "I divided both sides by 0.15."  Or, they say "I used p=mv.".  Um, I know -- you just wrote "p=mv," you don't need to tell me again.

Ask: "Explain in two tweets how to solve the problem."  I propose that students have a friend at our rival high school who needs help, saying via twitter that they don't know what to do.  You have to help.  You get to communicate in only two tweets -- that's two sets of 140 characters each.

The secret to teaching students to write is to clearly define an authentic audience.  They know without me saying anything that an online friend doesn't want to hear the poor annotations I've described above.  They want to hear simple articulations of principles:

Spring energy becomes A's KE. That gives A's speed and momentum before collision. P conservation gives the blocks' speed after collision. 1/

Now, blocks are a projectile. Vertical kmatics gives time, d=vt gives distance since horizontal v doesn't change once blocks leave table. 2/

And this explanation is a strong response to the AP Physics 1 question, "Explain how you would calculate the distance block B travels after it leaves the table."

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