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14 December 2013

If you sound like a lawyer, you're wrong

This regularly-uttered aphorism applies in so many physics teaching situations.  While my original use is lost in the mists of time, I suspect it involved me explaining why a student can't have more points on a test problem:

"Mr. Jacobs, the problem doesn't explicitly say that the car's acceleration is constant as it slows down.  So, in theory, it could have sped up first because it hit an ice patch on a downhill, then the velocity time graph could curve like so."  

No -- the problem says the car skidded to a stop, your graph makes no physical sense even given your crazy assumptions, this unit is all about making simple, reasonable assumptions about motion.  Still don't believe me?  You are making squirrelly arguments worthy of Antonin Scalia.  And if you ever sound like a lawyer in physics class, you're wrong.

But this phrase isn't just a pithy way of shutting up a student whining for grades.  

It also provides students guidance about the level of analysis necessary on a problem:

"The problem says the tabletop is 'smooth.'  But that doesn't necessarily mean no friction -- there's no such thing as exactly zero friction force.  The lack of explicity means that I must assume a friction value of reasonableness.*  So I'm going to assume mu is smaller than the smallest mu we've dealt with in class, even though that makes the problem very complicated algebraically."  

If you sound like a lawyer, you're wrong -- just interpret "smooth" to mean "friction is negligible."  Physics problems require reasonable assumptions, but they don't require tortured assumptions.

* Yes, for those readers who don't actually teach high school, students write like this.  No kidding.

This magic phrase can guide students to the depth of explanation necessary:

"Since the equation includes a squared term for the x-axis variable, the graph should look like a parabola.  But, a parabola can look like a line if the axes are zoomed in enough or on certain parts with less curvature than others (as we discuss on the first day of calculus).  Since the graph doesn't explicitly indicate the range of values for which we're graphing this data, I can't say for sure what the graph would look like.  It could be straight, it could be concave up -- assuming that mass can never be negative, which is a generally valid assumption except in some very special cases in extreme cosmology."  

Um, your first line was sufficient.  The rest sounds lawerly.  Once you start sounding like a lawyer, you're thinking way beyond introductory physics and the rough but accurate predictions we can make.

And finally, the phrase applies to teachers, too, as we grade papers -- if we have to make the lawyerly argument for the student, his answer is wrong:

"Hmm, I asked the student to draw and label forces acting on a block sitting still on an inclined plane.  This guy drew the weight, and a force labeled "Fn" straight upward.  Well, that up force could be the vector sum of the normal and friction forces; I know these are generally drawn separately, but if we were to add them together we'd get a resultant force of the incline straight upward.  And his Fn certainly could mean the "n"et force applied by the plane.  I'll give that credit."  

NOOOOO!  It's the student's responsibility to communicate correct physics.  Every time you worked on an inclined plane problem in class, and on every incline problem in your textbook, the normal force and (if applicable) the friction force are labeled separately.  And the conventional meaning of "Fn" in this context is the "normal" force.  Awarding credit requires you believing that your student (a) developed on his own the idea of labeling a single force to represent the vector sum of the friction and normal forces, (b) recognized the correct direction for that force, (c) redefined in his own mind (but not on the paper) the conventional meaning of Fn, and (d) communicated all these unlikely thought processes sufficiently.  Come now... isn't it far, far more likely that this student just automatically drew the normal force straight up rather than perpendicular to the incline?*  When you sound like a lawyer, you're reading way too much into a student's response.  Count it wrong and move on.  

* And in the one chance in a bobzillion that the student points out that he actually intended all of (a), (b), and (c), then you point out (d):  communication is part of physics.  If you have to explain your answer to me orally after the test is over, then the answer is wrong.  The time for communication is in writing, on the test.

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