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13 May 2016

Mail Time: Why is there not a lot of rotation on AP Physics 1? (Or, is the seeming dearth of rotational questions a valid perception?)

Reader Sara Rutledge asked this question in the comment section of the post in which I linked to the solutions to the 2016 exam.  I think it deserves its own post, so as not to be lost in the depths down the page...
My students commented that the exam had very little rotational motion beyond the FRQ with rotational kinetic energy. Is there an effort by test writers to match up the percentage of objectives on a topic with the percentage of questions on the exam? We had spent a lot of our review time on torque and conservation of angular momentum, so students were surprised that the exam focused on linear concepts and didn't seem to have a balance. Do you have any insights/advice on this? 

Sara, my understanding is that what we think of as "topic areas" are virtually irrelevant to the distribution of questions on the exam. Questions are distributed by the combination of "Big Idea" and "Science Practice." 

For our purposes, that means a problem relating torque to angular acceleration is EXACTLY EQUIVALENT to a problem relating force to linear acceleration. Conservation of angular and linear momentum are equivalent in the development committee's eyes, as long as the questions use the same science practices. 

Now, I'm sure there are discussions among the committee about balancing linear and rotational concepts a wee bit. But I'm not privy to those conversations. In terms of the goals of test writing, an exam could in principle be entirely linear, or entirely rotational, and still be considered a valid exam. 

As always, I take the most inference from actual, authentic, released items. And in the first two years of the exam, those released items are more heavily linear than rotational. 

Now, we haven't seen the multiple choice. I'm personally skeptical that there weren't at least a couple of rotational problems on the multiple choice. I think students see what they want to see: they wanted a torque or angular momentum problem, didn't get it on the free response, so likely ignored or forgot that it showed up in the multiple choice. 

That said, unlike the old Physics B percentage distribution of topics, it's quite possible that torque and angular momentum were in fact a negligible portion of the AP 1 exam. 

This year. :-)



  1. Thanks for the insight, Greg, about Science Practices. That makes sense and it certainly encourages us to make analogies between linear and rotational motion all along the way.

  2. When will the cut off raw scores for the 2016 exam be published? I would like to start grading my students following this scale.

  3. Not sure... they're probably pretty similar to the 2015 scores, though, with 70%/55%/40% the cutoffs or thereabouts. You can use those numbers in class without issue.