I had something of an epiphany as to when it is appropriate for me to use direct instruction in my classroom.
The modeling approach to physics has been a wonderful tool to give students real experience with the physics we are learning. Specifically they observe that the velocity plot of an object traveling down a ramp is linear, rather than memorizing this fact. They plot the position vs. time, and are able to explain the behavior of the graph using both their prior knowledge of algebra AND the observations in lab.
At the end of the lab students were able to construct mathematical model of the velocity vs. time from their data instead of being told v=at + v0 . Further more, my students were able to algebraically derive the equation x = 1/2 at2 + v0t + x0 . Once this has been established, the real physics is in making sure students are clear on all the ways that the graphical, verbal and mathematical models are related.
In my opinion, APPPLYING the equations of the CAPM (constant acceleration particle model) is actually not learning physics. It is what one is able to do successfully (meaning obtaining numerically correct answers, and being able to explain why such answers make sense when compared to what the verbal model and the plots predict qualitatively) once the physics as been learned.
Having concluded this, I realized I have nothing new to teach the students about the equations of physics. We simply need to provide them with an algorithm for solving a problem once they see the relevant physics in the problem. More speficially, the physics in a constant acceleration particle model word problem has already been “discovered” by the students. What they need to know how to do is identify how this physics is specifically connected to the problem in front of them.
To this end I gave them a 25 minute (with a break midway) direct instructed explanation of the algorithm I use to solve word problems in physics.
In short, direct instruction works for me when I am showing students how to follow recipes, NOT how to think.
Of course the can of worms this opens for me is… do story problems in the CAPM of physics actually have any pedagogical value?