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26 October 2016

Mail Time: "Normal force", "Flex force," or what?

Elizabeth Pate posted a comment asking about the reactions of AP readers:

I have a question about FRQ scoring. I use modeling instruction in my classroom so a few vocabulary terms that I use are different from a traditional physics course. For example, my students would call the normal force a flex force and the net force an unbalanced force. I do teach them both terms but I find they usually prefer flex and unbalanced force to Fnet and Fn, will these cause a problem for graders reading their FRQ's?

Reading AP exams is kind of how I learn about new trends in physics education.  Last year I learned about a commercial device with two (visible) lasers and photoreceptors; when the beams are broken by a moving object, the device calculates speed.  I'd call it a visible photogate.  I forget what it's actually called, but enough students used this device on their lab problem that I found out about it.

So, "flex force," eh?  That makes sense.  Never heard it before.  How would I react to it as an AP reader?  It depends on the rubric, and how the problem is phrased.

When the test simply asks for a labeled free body, we are usually quite generous about those labels.  My quintessential example was a few years back when students had to label a buoyant force, the force of water on a cup.  We accepted "buoyancy."  Then we accepted the misspelled "boyance".  Then we accepted "bouncy," because it made sense in context.  A rumor was spread that someone accepted "BeyoncĂ©," but that's unconfirmed.  :-)

So would I accept "Fflex" on a labeled free body?  Possibly, especially now that I've heard from you that "flex force" is modeling vocabulary for "force of a surface on an object, acting perpendicular to the surface."  

I always train the students to define their labels.  Don't just say "Fn" -- say "force of the road on the car" or "force of the scale on the boy."  Then even if someone misreads the label, or if the label is unintentionally ambiguous, there's no issue.  

For example: we've never accepted "G" as a bare label for the gravitational force, even though some texts and teachers may teach that.  Why not?  Because "G" has a well defined conventional meaning: it's the universal gravitation constant, 6 x 10^-11 N*m^2/kg^2.  

But a student who labels the diagram with "G" and then says "G: force of the earth on the car" earns full credit.  Oh, and he can deal well with Newton's Third Law, too, but that's for a different post.  :-)


1 comment:

  1. Flex force is interesting, because it gets across the underlying physical origin (compression) for the 3rd law reaction from the surface. Its only flaw is that Ff is usually friction so it really does have to be spelled out. That said, its advantage over "normal" is that many, perhaps most, students don't know that word as a specific mathematical term. (Is it on your fact list?) Their first instinct is that it means some regular old force. I sometimes start them off using the perpendicular symbol for it, then get them to use Fn as a synonym. Describing Fn as the "perpendicular surface force" also reduces errors when drawing it on an inclined plane.

    I prefer "net F" over "F_net" because the net force is a sum, not a separate force. I also see many instances where students were taught (in a mediocre physics class) that "Fn" is short for "F_net" and make spectacular errors when they confuse the two. Even worse, some were taught to put Fnet or even m*a on a free-body diagram as if it were a force to be added to the others.

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