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## 03 November 2014

### Direct Measurement Video assignment: Einstein Rides the Gravitron

I've discussed "Direct Measurement Videos" before, in the linked post.  These videos are wonderful, because instead of a presenting a sterile "imagine this situation" type of textbook problem, the situation doesn't have to be imagined -- it's right there on the video.

But what exactly do I do with these videos?  I've been asked that question a number of times.  Here is my AP Physics 1 class's assignment for Monday, verbatim:

In the video linked above, an Einstein doll on a rotating platform appears pinned to a wall, as shown in the screenshot.  As the platform slows its rotation rate, Einstein remains pinned in place until he eventually falls.

You are to determine the maximum coefficient of static friction between Einstein and the wall.  Justify your answer thoroughly – this means you have to explain not only how you solved the problem, but how you obtained or estimated the necessary data from the video in order to solve the problem.  Start with a free body diagram of Einstein, obviously…

This worked out better than I could ever have imagined.

See, I'm dealing with a number of students who are not appropriately connecting mathematics to physics.  They want to explain results without reference to equations; they want to do calculations (both in variables and in numbers) without any verbal explanations.  When they're asked to explain a calculation, they tend to explain the algebra ("I subtraced T from both sides to get T = Fnet +mg") rather than explaining where the equations come from, and where the values they need could come from.  These deficiencies are hardly unusual in an AP class; but I am struggling this year to bring my class into a real understanding of quantitative-qualitative translation.

This video assignment seemed to bring out my students' best.  Most of the class made the free body diagram, set the friction force equal to Einstein's weight, and set the normal force equal to mv2/r. They knew from practice that the speed v can be written as (2πr/T).  They used Ff/Fn to solve for the coefficient of friction.  They made a table of values to plug in, and got a reasonable coefficient.  Great.

But then something beautiful happened... virtually all my students, even the ones who had been struggling, wrote me crystal clear explanations to follow up on their mathematics.  They told me exactly what I told you in the previous paragraph -- sometimes in the very words I used.  They explained how many frames were in a revolution, and how they calculated the time for one revolution just before Einstein dropped.  (Or, how many frames were in a HALF revolution before the drop.)  They either explained that they estimated Einstein's mass, or that they noticed that his mass canceled out of the equations they derived.  They explained how the radius of curvature was determined from the video.

In other words, they completed the most thorough quantitative-qualitative translation that they've done all year.  Somehow, my students have been unwilling or unable to describe the process behind a calculation from a textbook-style problem.  The video brought out the best in them.  Why?  I don't know.  But I like it.