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Can force be transferred?

September 25, 2012

Yesterday we started the chapter on forces. The first big question to come up in this chapter is ‘Can force be transferred just like energy?’ I usually address this question in the following way:

  1. I acknowledge at the start of the activity that this is a significant question and I tell them that we will decide our answer by the end of the first activity.
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  2. While working through the activity, at least half the students start leaning towards ‘no’.
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  3. At the end of the activity I ask students to write in words how they would explain what happens when you push a cart with your hand. I ask some students to explain using energy ideas (which we developed last chapter) and I ask some students to explain using force ideas and taking the perspective that force can be transferred. (I also sometimes ask some students to explain using the perspective that force cannot be transferred but we often don’t have time to discuss these in the same class period.)
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  4. I ask students what are the similarities and differences between the ‘energy transferred’ explanations and the ‘force transferred’ explanations. Students usually point out that these explanations are the same except that the word energy has been replaced with the word force.
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  5. As a class we decide that we don’t want force and energy to be the same thing (that would be redundant) so we decide that force cannot be transferred.

 

Yesterday, when students shared their different explanations with the class, one group came up with an explanation that force is transferred but that once the force is in the new object it becomes something else like momentum. They made an analogy to our ideas about energy and how we call the energy ‘mechanical energy’ while it is being transferred but once the energy is in the object we call it ‘kinetic energy’. So force is transferred but it doesn’t stay as force, it transforms into something else. The students did a really fantastic job of articulating their ideas on this subject.

I find this to be an absolutely wonderful idea and a fascinating example of students using their ideas about energy to build an idea about force (and momentum). At first glance I can see how this conception of force leads to many of the same conclusions as the more traditional conception of force (that force is an action, not something that gets transferred). If two objects collide, one object will transfer force to the other object resulting in a decrease in momentum for the ‘source’ object and an increase in momentum for the ‘receiver’ object. This is the same conclusion I would arrive at by applying Newton’s 3rd law and F=ma.

At first it seems like this conception of force would run into trouble when attempting to explain two-dimensional motion where vectors become important, but we could simply define this ‘force’ that gets transferred to be a vector which can differentially affect momentum in different dimensions. If we play this game out will this conception of force evolve into our traditional conception of momentum? It looks like it at the moment but I can’t say for sure. This idea is really challenging my ideas about the differences between force, energy, and momentum. What (if anything) would we lose if we adopted this conception of force? I can’t answer this question off the top of my head.

An Instructional Note

During class I shifted attention away from this alternative idea of what it could mean for force to be transferred. I had laid out an instructional roadmap in my head for getting students to decide that energy and force are different concepts. This alternative conception didn’t fit into my map and so with class time running out I focused the discussion on the student ideas that aligned with my roadmap. Brian’s discussion of SV’s pursuit of the normal force bridging analogy has helped me reflect on the instructional choices I made during this group discussion. I’m excited for tomorrow’s class to let students know how fascinating I find their alternative idea about force transfer and that their idea challenges my own understanding and my own ideas about force and energy.

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