Do we teach force decompisition too early?
I want to use this space to think about a conversation I had with Mike Shatz, a professor at Georgia Tech, with whom I’m collaborating to bring computational modeling to my 9th grade physics classes.
Mike and one of his graduate students led a 2 week workshop for the Atlanta Public Schools this summer, and before that, they wanted to talk to me about my approach to teaching. I described the modeling process I follow in considerable detail, and shared with them some of the modeling materials for the first few modeling units. They thought that this approach might be a good way to structure part of their workshop. Important note: Mike and his student were not teaching an official modeling workshop. In fact, this was a workshop for physical science teachers, and they were using their interpretation of the modeling methods to give these teachers some understanding a more student centered instructional approach like modeling.
Mike and his grad student, Daniel Borrero worked through the following units, in this order” Constant Velocity, Constant Acceleration, Balanced Forces, Unbalanced forces. After this, Mike had a pretty interesting argument for me, and a few other observations.
Mike told me that he thinks Modeling teachs force decomposition too early. As part of the balanced forces unit, students are interoduced to how to add individual forces to compute the net force. Shortly thereafter, in the unbalanced forces model, students learn to solve problems like a block sliding down an inclined plane, and one method for solving these problems is to tilt your axes and so that the x-axis is parallel to the surface of the ramp, and then decompose the gravitational force into two components, one perpendicular to the ramp (which is counteracted by the normal force) and one parallel to the ramp (which causes the block to accelerate down the ramp).
Doing this decomposition is both conceptually and computationally challenging for students, especially for those with weak mathematical backgrounds, and Mike’s critique is all this takes away from the big idea that you’re trying to teach—the object accelerates in the direction of the net force, and students are getting all bogged down with components and trig functions, and might very well lose sight of this.
I think Mike’s alternative would be to spend all your focus on understanding that it is the net force that dictates how the motion of an object changes, doing a lot of lab work and computational modeling using VPython to understand this better, and not getting bogged down with trying to describe, decompose or calculate the individual forces that make up the net force until you’ve mastered this idea.
There are parts of this argument that I find very compelling—and in an upcoming post I’ll share a activity the Mike and Daniel did with their workshop, but with only a couple of weeks before the start of school, I don’t think I’m going to be able to fully revise my curriculum to take these ideas into account.
So what do you think? Would mechanics be easier to understand if we pushed off force decomposition and just focused on the net force?