Day 68 – Phrustrating Physics

Physics 11 – Today we had two main tasks. First, the students wrote a quiz with 4 different learning objectives: drawing force diagrams, determining if forces are balanced or unbalanced, knowing what vectors are, and Hooke’s Law. In general the quiz wasn’t done very well and since there were four learning objectives, class overall averages dropped by almost 5%. I think many students simply did not study.

We also finished the Fnet lab and analysis. I’m very fuzzy on whether it was worth it.  These videos are best used as secondary teaching tools, but I used it as the initial tool since we don’t have good (ie any) lab equipment for Fnet experiments. I really hope to get some probeware next spring so we can do labs like this hands-on. Doing this exact lab with probeware means that:

  • lab software shows the x-t and v-t graphs instead of abstract numbers and calculations
  • each group can ran multiply experiments with relatively quick data collection
    time can be spent on analyzing and thinking, instead of calculations

Day 73 – Unbalanced Forces


Physics 11 – Students were applying their understanding of force diagrams, Fnet and unbalanced forces to problems today.  I usually don’t get too prescriptive on problem solving techniques, but I spent three days emphasizing #1 and #2 in the photo. As all teachers know, kids really resist doing things they think are not necessary. Unfortunately for them, skipping steps 1 and 2 will almost certainly result in mistakes even for the smartest kids with the most intuitive understanding of physics.

The Big Tip for Physics Teachers

Ok, so we all know that kids try to skip steps. My students would often mix up writing out an equation for the unbalanced forces (Fnet equation, Fnet = ….) with Newton’s 2nd Law (Fnet = ma).  Here is the new thing I found out this year: after kids draw and label their FBD, they become paralyzed on the next step because they realize they don’t know what all the forces are.

I used to think that they don’t write out Fnet = .. + … + … because they were trying to skip a step. What’s really happening is that when they start to write out Fnet = …. what they’re expecting is that they should already know everything on the right hand side of the equation.  They often don’t, and this becomes a discrepant event for them.

There is a strong link between this and a phenomenon in math.  In math, students often have a difficult time understanding the “=” in an equation.  Many people don’t see equations as being a statement of equivalence.  Instead, they see an equation as a question on the LHS (what is?) and the answer on the RHS. This is what happens with Fnet. The intuition is to have an answer for everything on the RHS.  Note: just because students can give a definition of what the equals sign means, doesn’t mean they treat it like that when working with math or physics.

Day 66 – Lab Wrap Up



Physics 11 – Today we finally debriefed the lab results.  There wasn’t a lot to say at this point because we had spent so much time talking about things just to get here.  I really wish I could see how other modellers handle this kind of thing, I wonder what their whiteboard meetings look like?

A few things I would do next year, if I do this again:

  1. Force all groups to graph F vs a.  While we agreed that a vs F made more sense with acceleration being the input, the analysis for realizing a general model from this is not obvious.  By graphing F vs a, it’s easier to see that the slope is the system mass
  2. Have each group explicitly write down their system mass in kg while collecting data.
  3. Have a clearer discussion on how Fg = Fnet. I mentioned it, but I don’t think it resonated with the students.

Would I do this lab again?

There are three major problems with this lab. First, it took a heck of a long time. Second, not all the kids seemed to get the big picture.  They got some data, calculated numbers that Mr. Smith told them to, and graphed them like Mr. Smith told them to.  Some kids don’t fully appreciate the thought they put into their work.  When they ask me questions like “what do I graph?”, I’ll answer with things like “well, what relationship are you trying to find? What should you graph? What should your independent variable be?”

My last statement above highlights the third problem.  If they kids graph a vs F, which makes sense, then there is no obvious physical explanation for the slope. I have to explicitly tell them to change their equation to a =… (why?) or to purposely graph F vs a.

On the positive side, 2 of my 3 classes categorically thought the lab was good and recommended for next year’s class to do it. Several kids did have a solid A ha! moment, which is always good.  Others said that playing with the video and analyzing the data made them appreciate how forces have a direct affect on acceleration.

I wonder if a more qualitative lab could replace this one? Something like what Josh Gates wrote about in the Physics Teacher (2014). I currently can not do exactly what he does because I don’t have the sensors, but it could be something to aim for.


Gates, J. (2014). Experimentally Building a Qualitative Understanding of Newton’s Second Law. The Physics Teacher, 52(9), 542–545.


Day 58: Unbalanced Forces Results


Physics 11 – Today the students wrote up their results from their lab for Newton’s 2nd Law. Many groups were getting close to developing a model for Newton’s 2nd Law: Fnet = ma.  Since the input for the lab was a force though, the students graphed a = Fnet/m

Once the slope was written as a fraction, the students were able to recognize that it was the inverse of the mass.

I have two classes of physics 11.  During the first class, I walked along a lot and spoke with students as they worked.  As a result, by the time we got to share the whiteboards, almost everyone had the same board. For example, I challenged groups to find better symbols to use, rather than x and y.  The second class I helped a lot less and the following discussion was better.

After our discussion, the students applied their new model to the situation of someone standing on a scale inside an elevator.  This allowed for some thoughtful thinking on adding forces together from well drawn force diagrams.