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10 December 2012

Making students write facts of physics in every answer

I've discussed how "justify your answer" means to use either a fact, equation, or a calculation to support an answer.  All year in 9th grade conceptual physics, I've handed out printed sheets listing the appropriate facts and equations that can be used for justifications.  Still, a significant subset of students have disappointed me with their justifications: they make up facts, misstate facts, or sometimes just write any old fact, whether or not it has any relevance to the problem at hand.  

I got sick of nagging my students to use facts of physics correctly.  Taking a page out of Jen Deschoff's bag of tricks*, I decided to have the STUDENTS grade assignments for completeness.  Then, the class could have an additional day to check their answers with friends before turning in the final version of the assignment.

* Mixed metaphors are legal in physics blogs

For the past two weeks, I've started class by collecting a problem set and redistributing it to the class.  Each student gets a red pen and is asked to check boxes in a rubric that says:

* Is a fact written nearly word-for-word from our sheet?
* Is the fact relevant to the problem?
* Is there a sentence showing how the fact applies to the problem?

We apply the rubric above to each problem on an assignment.  If even one item is missing on one problem, I take off substantial credit; but I award some credit on each set to those who do everything right.

In the first day or two, lots of students lost credit.  But, it wasn't big bad Mr. Jacobs taking off the points -- their own classmates were the ones checking the facts.  I hold everyone accountable for their grading by asking them to initial the page they grade.  (That means I know who graded what, and I can have a word with someone being too strict or too lenient.)  I'm finding, though, that I don't really have to follow up at all.  The students are more careful about grading than I am.  One even asked, "Do I need to take off for spelling?"  Miracle of miracles, after two days I found everyone writing out facts clearly; also miraculously, everyone was doing better on the problems, because by simply being forced to pay attention to writing a proper fact, the logical connection to the correct answer became more apparent.

Perfect for studying position-time graphs

We spent a week working on just position-time graphs.  The most common issue at the beginning of this unit is a failure to separate the "picture" of the graph and the motion represented by the graph.  For example, students commonly say that a graph with negative slope must represent a cart going down a hill, because the graph looks like a hill.

I only had one student make that mistake this year.  Why?  Because everyone had to start every problem with something like "A position-time graph sloped like a back slash \ means the object is moving away from the detector."  It REALLY takes some serious cognitive dissonance to tell me that the cart is thus rolling down a hill, especially since the problem starts with something like "...the detector is pointing north."  

And velocity-time graphs

After a week, we move to velocity-time graphs.  Usually the abstraction that the graph represents motion on a line is clear by the time we move on; the biggest issue I've had teaching velocity-time graphs is students not frickin' paying attention to whether a graph is position-time or velocity-time.  In past years, I've had people telling me for v-t graphs "the cart moves south because the slope is like this \" even after they've made the same mistake countless times already.

No problems yet this year... because of the student grading.  The second bullet point on the rubric says "Is the fact relevant to the problem?"  I talked to the class ahead of time about how facts for the wrong type of graph are by definition not relevant to the problem.  Meaning, if you use position-time graph facts for a velocity-time graph, I'm not marking you wrong, your friends are; and you're losing not just one point for a "good try, honey," but substantial points for laziness.  

Would student grading for written-out facts work for juniors and seniors?

Not sure.  I've never tried this approach with upperclassmen, because some might see it as busy work, and might rebel.*  But the positive correlation to comprehension is so incredibly strong, that I might suggest giving this sort of approach a try.  Let me know if you do.

* Freshmen, I've discovered, are poor rebels.    

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