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01 April 2012

Experiment: density of mystery fluid, and the audience for a lab writeup

Materials for the "density of unknown liquid" experiment

Fluid mechanics first entered the AP physics B course description in 2002.  That year, the laboratory question (#6, I don't have a legit link, but it's easy enough to look up) asked students to determine the density of an unknown liquid by submerging a mass on a spring into that liquid.  

My own classes did a version of that problem on their trimester exam.  The ones who got it wrong -- usually by conflating the density of the liquid with the density of the submerged mass -- did an exam correction on which they described the procedure correctly.  

And then this week, I had everyone actually, honest to goodness, do the experiment for themselves.  I've often suggested that the AP exams since 1996 provide a wonderful laboratory guide, if you can improvise a bit.  Pick a lab-based question, set it up with whatever equipment you have lying around, and there you have a college-level physics laboratory exercise.  I took my own advice and tried out this new experiment.

I gave each group a beaker of fluid and a mass.  (You can see the cubical masses in the picture -- they all are made of different materials.  I just dug them up in an old storeroom.  I have no idea where they came from.)  The groups were encouraged to pick the spring of their choice.  They could use any other equipment they wanted, including a balance scale; the only action I forbade was to directly measure the mass of the fluid in the beaker.

Now, most of my experiments, and many AP lab questions, call for a linear graph, a best-fit line to copious data on the graph, and interpretation of the physical meaning of the line's slope and intercept.  This particular experiment doesn't lend itself well to a graph, at least not the way it's presented on the 2002 exam.  So I came up with an alternative approach to the writeup.

I used a true mystery liquid rather than water.  I won't reveal what I used (because my students occasionally read this, and the writeup isn't due 'til the end of the week), but I don't even know its density right now.  We're going honestly double-blind here.

I'm asking each partnership to write up one typed page describing their results.  I give no specific instruction other than to imagine that they've been hired to determine this mystery density, and that their financial well-being depends on the quality and accuracy of their work.  They get one shot to impress their potential client with their writeup.

That "client" is Peter, the captain of our USIYPT research physics tournament team, and the rest of his class.  I will collect the typed pages with no names, and hand them to Peter and the research students.  They will rank the papers from best to not-so-best.*  I'll assign grades based on the rankings, and give a prize to the winners.

*Donald Trump would say "worst."  Why is it that teachers tend to get in trouble for such language, while Mr. Trump is lauded for his bluntness?

Sean measures the extension of the spring
Sure, this is a nice cutesy little game.  But there's a real message here.  Too often when students are asked to describe the results of an experiment, they use stilted, overly-formal language that stifles meaning in favor of big-arse words.*  I want them instead to write informally for an audience at their own level of physics.  A major obstacle to improved writing is the faceless audience.  High school students have never published anything; their understanding of a paper's audience is poor even in English class, let alone in a subject where they struggle both with the content *and* the writing skills.

"In this experiment, the experimentors carefully and consistently used a decimal-labeled wooden shaft to record precisely the extent to which the PASCO brand spring was extensively extended.  We ensured safety by wearing goggles and grounded electrical outlets." 

So, I put a face to the audience:  Peter.  Everyone knows Peter.  They talk to him in normal language.  They are not in fearful awe of him, but they are all quite clear that he and the research team have no use for incorrect physics.  (I wonder where they got that from...)

I've never done this particular exercise; but I have had students write with a named fellow student as their audience.  Their writing doesn't become perfect, but some of the filler gets filtered out.  And so we focus on the physics... which is what I want, anyway.

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