Day 62: ‘sigh’ Action-Reaction makes me a bit sad.

Advanced Physics:

We had a brief discussion about forces that included a definition.  We (ok, me) defined a force as an interaction between two objects.  We talked about the symbol (Fsubscript (on, by), the units and how we measure.  We then moved into an ILD looking more closely at comparing the size of force Object A exerts on Object B.  I’m sure most of probably do this in some way, shape or form.  The version I use had ten situations, here is a sample:

Screen Shot 2014-12-01 at 9.44.41 PM

The students choose A (object A exerts the greater force), or B (object B exerts the greater force), or C (they exert the same size force.  I also ask them to provide a reason.  I use the clickers in anonymous mode so we can get a feel for where the class is.

To check the situations, I use two Vernier WDSS as the force sensor.  They work much better than the dual range force sensors I used to use.  Here is a screen shot of on of the tests (and how I phrased N3L):

Screen Shot 2014-12-01 at 9.52.33 PM


A fair number of my students, almost on cue, say “Oh yeah, for every action, there is an equal and opposite reaction”, which makes me sad.  I just feel this statement of N3L leads to a bunch of misconceptions.  Sure they can recite the 3rd law, but have no concept of it.  For example, I walk over to a student, a give him a back hand on the shoulder (no worries, it’s a soft back hand), then the student gives me a back hand on my shoulder right back.  I ask the class, “What was the action?”, Again on cue, a fair number say ‘you hitting… and the reaction was … hitting you back’. WRONG< WRONG< WRONG.

That is one of my issues with the Action-Reaction statement… it implies that time delay… first this, then that, when in reality the two forces are exerted at the exact same instant as shown over and over again by the ILD.  I also don’t think that action and reaction makes one think of forces, but it’s all about forces.

I could go on with this rant, but I won’t, just know that ‘action-reaction’ makes me a bit sad.

General Physics:

Today we WB’ed the ten ‘stack-o-graphs’ the class had as practice over the weekend.  As an added treat, after each group was finished presenting, I had one member of the group walk it or give me a real-life -in -a-car situation that would produce the graphs.  We sent them home with a one more to practice with… a ramp set-up from the Roll-Ball App.  This takes it one step farther… decide what the motion is, then create the stack-o-graphs.


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