Monday, July 7, 2014

Forcefully Losing My Marbles

The marble "discrepant events" today were interesting.  My teacher brain predicted them correctly, but using energy rather than kinematics.  It would have helped me wrap my head more around how to use them in the acceleration unit to see how the discussion at least started to play out in student mode.  This will probably get mentally filed under "fun if I have time" or maybe "save for energy unit warm-up".

The theme of today was forces (mostly static) and I think I've managed to pre-absorb more modeling ideas about this than I'd realized.  My students know that a force is an interaction between two objects, and name it F^{type}_{actor}_on_{victim}.  We've worked our way through a (slightly materials-limited) version of the bridging analogies for normal force (is there a reason we're not calling it the normal force yet?  will we ever?).  And we do a whole-class version of the Weight vs. Mass lab with which we ended the day.  And I've picked up along the way the distinction between inertial and gravitational mass, and the distinction between the 9.8 N/kg gravitational field strength and the 9.8 m/s/s gravitational acceleration, and I try to help my students see the distinction.

On the other hand, I've tended to be very loosey goosey about force diagrams, especially with my ninth graders.  Which has, I think, made components I lot harder for them to grasp.  It felt a bit like jumping in the deep end of a very cold pool to do Unit 4 WS 1 and start doing slanty, component-y force diagrams on only #4.  I have tried to teach graphical addition of vectors and force components, and I think I've mixed it in with the force vectors in such a way that students' confusion ends up compounded.  (At least based on my very high rates of retesting my components objective.)  I'm curious to try the physical model of #8 to help with building students' concept of components -- my approach to components in the past has, I think, been was too abstract.  I'd love to give each group of students two strings, a hooked mass or two, and have them work through the example Laura showed us for themselves.

I've typically done the weight vs. mass lab very quick-and-dirty by having each table pair weigh one hooked mass and plot the observed force against the nominal listed mass on a projected graph that everyone used to recreate the graph for themselves in their own lab book and come up with a best fit line.  We usually get something quite close to 10 N/kg, and then use that line to make the F^gravitational_earth_on_object = 10 N/kg * mass equation.  The advantage of this approach is that it's fast (20-30 minutes for the whole thing), and each student pair gets to make one measurement for themselves.  The disadvantage is that they don't experience for themselves a range of masses and notice the different spring compressions / extensions.  On the other hand, they've already done that with a different Hooke's Law lab... Not sure if the extra time gives a big enough pedagogical payoff to do it the way we did today.  Will see how the semester plays out.

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