Day 63: A failed attempt to develop balanced vs. unbalanced forces (and N1L)

Advanced Physics:

My mod one class was really awful today, or rather, I was really awful in mod 1.  To begin the class we played with balloon-o-copters to identify third law pairs.  That went well enough except for one little item…last year we purchased  about 300 of them for a very cheap price, thinking we’ll give one to each student, they can play with it and use it to teach others about N3L.  The only problem is that they really did not work so well, or at all.  But that’s not the really awful part.  My usual sequence is to develop N3L and the paired interaction, then explain that to explain the motion of one object, we need to look at the forces acting ON that object, not the forces exerted by that object, so one side of the force pair.

Over the course of my career, I have tried a wide variety of activities to develop the concept: guided discovery with large chunks of dry ice, guided discovery with bowling balls and pvc mallets or rubber mallets, hover pucks, the bowling ball grand prix.  I was never really satisfied with how the activities went and my post lab discussion to bring it all together.  For the last several years, I have used this activity from Kris Troha on Twitter a year or two back.  Essentially it uses the set-up shown to connect balanced forces to constant velocity and unbalanced to constant acceleration.  I like it because it is easy to set up and does exactly what I want it to.  One draw back is that it is not a kinesthetic experience for the students, and it does not really develop the concept of inertia as the bowling ball (and replacing it with a basketball) does.

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Well, on Sunday, I read a pretty good article in the most recent issue of The Physics Teacher by Joshua Gates, called Experimentally Building a Qualitative Understanding of Newton’s Second Law, you should read it.  It describes an experiment about using a WDSS and a 2 pulley Atwood Machine.  I liked it and thought I could use it to develop the balanced vs. unbalanced forces, add a motion detector and connect it to motion.  I thought through it very carefully last night and typed some things out (no handout though) and went for it.  My HUGE mistake that I figured  out before I made a really HUGE mistake was adding a second WDSS as the other mass.  I showed the mass (one wdss) moving vertically and measured both forces… yep  reading the same value so balanced…. motion detector showed constant velocity … done.

UGHHHHHHH I hate it when I screw up. I knew it right away as I was writing it on the front whiteboard.  In my mind, all I really showed was that the tension is the same in the string…. I never defined my system as one of the WDSS’s and looked at the forces exerted on it (as our system) as I had talked about merely 10 minutes before this.  Constant velocity because the forces acting ON the system are balanced… namely Fg(WDSS, Earth) and Ft(WDSS, String). NOT the two blasted Ft values!!!!

Luckily the bell rang so I could re-group. I talked to one of the students in my class (OK, my daughter) and she did not even know it was a screw up… that does not make it ok, but she did say it was confusing.

So, what to do tomorrow… well besides owning my screw up, which I will do publicly in class, I’m going back to the cart with two tension forces, but adding two WDSS’s and a motion detector to provide proof.

SIDEBAR:  I can not find the link to the original twitter post, but if you want me to share the handout based on Kris’ idea, let me know.


General Physics:

A bit more practice on the Stack-o-Graph’s.  We tried to use the Roll Ball applet hosted on the Labout Loud site, but because it is an old version, the Java Security prevented us from having our students use it on laptops. Instead I used it on my laptop ( I fixed the java security so I could use it).  We took an assessment that I used the results from to put the students into groups for tomorrow —>Phan Cart activity.