Tag Archives: modphys

IB Physics is Too Hard

For several years, I have harboured the notion that IB Physics is too hard. I suspect I’m not the only one:

For a student, the difficulty of physics is perhaps easier to explain: the ways of constructing meaning are challenging, the models of knowledge demand careful study for mastery, and the assessments are notoriously hard to bluff your way through. For educators, however, these difficulties are part of the trade. Textbooks, lesson plans, and pedagogical approaches provide good ways for students to overcome these issues.

Even as an educator, however, IB physics still seems hard. Some people believe that the syllabus is too broad, encompassing too many topics like particle physics, wave interference, thermodynamics, and the greenhouse effect. Others point to the depth of understanding required to score well on the exams. However, I don’t think that either of these dimensions is sufficient to explain what is so hard about the syllabus.


I’ve added a third dimension to the “iceberg” of physics. Thickness is about the skills students need to acquire in order to do physics. To see this, let’s look at some examples.


Example 1: There’s a car on a hill, with lots of known properties.

  • Depth – what is the car’s velocity at the bottom of the hill? what is the efficiency of the car’s motion? if the car were to roll back up a similar hill, how far would it go?
  • Breadth – what would be the impact of a non-zero drag force? how does the calculation change if you consider the angular momentum of the wheels? at what rate is the car increasing the temperature of the hill?
  • Thickness – how do we know to use energy conservation? what does our model include? how can we understand the energy transformations within this model? how do we know if our answer is reasonable?

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Example 2: A past IB exam problem (above)

In this problem, students are expected to use the Rayleigh criterion to estimate resolution. It is a simple calculation that they have already done a few times, using a formula from the data booklet. This topic demonstrates both the breadth and depth (this is from a unit on single-slit interference) of the IB physics curriculum. It also demonstrates thickness: conceptually understanding what is going on here is very challenging. Although this appears on an exam, I suspect most IB physics students have just memorized how to use the formula.


Thickness in physics involves the skills, approaches, mindsets, and contextual knowledge that allows physicists to do physics. This includes things like critical thinking, building models, visualizing phenomena, drawing representative diagrams, working fluently with numbers, dimensional analysis, and using the right model at the right time.

IB Physics is hard because we neglect the thickness of physics, focusing on solving standard problems and preparing for tests instead of learning the skills of physics. And when students do well in this hard course, it isn’t because they mastered physics: it is because they did well on the test.

Let’s return to the tweet by @AfroRose_: “I remember when I was in IB physics, it was so hard. What’s crazy was in class I always got the answers right, but I couldn’t explain myself [emphasis added]”. Here is a student who studied hard and succeeded at IB physics, but never became competent with the skills involved in the discipline itself. For her, physics was deep, broad, and thin.

Over the past two years, I have tried to reconcile a deep, narrow, thick pedagogy (Modeling instruction) with a deep, broad, thin syllabus. There have been productive moments — the thickness is especially valuable in preparing students for their independent investigations — but any concession toward thick teaching comes at a cost in contact hours and, since the alternative is to prepare students directly for their exams, potentially a decrease in student grades.


How can IB Physics be more thick?

The AP and A-level boards have both done an admirable job in their past revision cycles of cutting down the amount of material students need to learn, that is, decreasing the breadth. The IB revision cycle sort-of-not-really did this, by reducing the number of optional topics, and shoehorning some extra particle physics into its place. A decrease in breadth is necessary because of the limited amount of time available for learning.

With a bit more time allowed for the development of skills with mechanics, for example, we could include system schemata, LOL diagrams, or graphical vector addition for forces. If we keep Feynman diagrams, we could have enough time to develop students’ understanding of them. With scaffolds like these, we could be confident that students will be better prepared to tackle unfamiliar, complex problems on exams. This could even allow for the use of open-ended problems. I think that would be pretty cool.

A change to a syllabus that seems to make it less hard will always be met with concern. I want to smart-up students, not dumb-down the curriculum. In any case, I wouldn’t be worried about blowback.

#modphys: English, revisited

Back in August, I wrote about my attempt to understand how English communication was getting in the way of measuring scientific reasoning skills. I assigned my students 20 of the questions from Lawson’s Classroom Test of Scientific Reasoning (CTSR), dropping the four linguistically toughest. Once they students had finished the test in the regular way, I demonstrated the scenarios one at a time to provide contextualization.

You can read more about the assessment and the results here.

There are two things that I’ve been meaning to adjust about my results. First, I forgot in my analysis that a correct response should only be noted as correct if the response and the follow-up “why” question are both correct. Thus, instead of a score out of 20, I should have a score out of 10. Second, I wanted to add a comparison to norms. In the graph below, the “Norm” line comes from a scaled (from 13 to 10) version of the results compiled by the Frameworks for Inquiry project. This line corresponds to the scores of 3800 American students from grades 10 to 12, and has meaningfully been connected with such things as Piagetian developmental stages.


I think it is clear that the blue line (my students in the normal testing situation) are pretty close to the norm data, while the post-demonstration results (in red) look to be quite different on this cumulative frequency graph. This should be taken as an indication that linguistic difficulties are an important factor in determining the score of the CTSR.

#modphys: Centripetal Force

Today we built up a model of the centripetal force. It was one of those “let’s trust that data, because I say so” models. The students have a lot more confidence when their data gives them a clear and straightforward relationship, so this was unfortunate.


The problem was with our apparatus. Lacking anything fancy, and also lacking the time to build Rex Rice’s excellent device, we did it with the simplest apparatus one could imagine. Here’s a schematic sketch:


There are plenty of problems with this. First is the difficulty in using it: one student holds the force gauge in one hand and also uses the straw to impart rotational momentum to the stopper. This is tricky! Second is the friction between the string and the top of the straw. Most significantly, however, is the difficulty (nay, impossibility!) of the hand in maintaining a constant stirring motion with the straw — this causes the force gauge to read a wildly-fluctuating value, and makes it very hard to ensure that the system is operating with a regular angular velocity.


My own pre-lab trials were iffy, but of course the students were much less careful with their own measurements. As a result, we got one F = kv^2 graph, one F = km graph, and one F = k/R graph. That’s just barely enough to convince that the form should be F = mV^2/R, but we were nowhere close being able to see that the slopes had physical meanings.

Next year, I’ll try to be a bit more on-the-ball about getting the device built. I’m also going to look into getting some more-accurate force gauges — these are okay, but don’t give the sort of accuracy we need. Maybe Vernier…

What IS Energy?

We have been developing the energy model over the past week. Today, the best thing ever happened: I heard “what is energy?” from three different people, and heard three different answers. To a colleague, energy is the ability to do work. To a middle school student, energy is a property of things that allows them to change how they are. To a high school student, energy is a quantity represented by the area on a force-distance graph that may change between gravitational, kinetic, spring, thermal, and other forms.


In fact, the success of this energy model makes me want to take a detour into our thermal physics unit. However, in order to spread out the workload for the Higher Level students, I think we will look next at projectile and circular motion.


Today’s whiteboards come from the inductions of the kinetic and gravitational energy forms.


#modphys: When it works

I had a good modeling class today, and I want to share what went well.

1. I had the students work in pairs to whiteboard solutions to the problems on worksheet 2 in the Unbalanced Force Particle Model [UFPM] unit. Unlike most of the worksheets, this one advanced some ideas through appropriate questions, so I didn’t need to edit it significantly.

2. I paired students with an eye toward challenging everyone. Two students needing reinforcement of basic ideas got the first problem, while the complex final problem went to a pair who were ready for it. At the same time, each pairing combined students who didn’t normally work together, which led to a lot ogood dialogues.

3. The solutions were all essentially right, and the students did a good job of explaining them. I even saw two students using my normal force demonstrator (below) and videorecording a pair of falling objects.


4. Even better, they did a good job of bringing forward some misconceptions. For example, many claimed that a passenger in a freely-falling elevator would hit the ceiling, and supported this belief through a chain of argument that rested on the axiom that more massive things accelerate faster when they fall. During this discussion, I had to say very little, as the students carried through the argument.

5. Twice I interrupted with impromptu demonstrations, the first (below) showing a beaker in a falling box, and the second dropping two objects of vastly different mass. I used a paper clip and a 1 kg mass — I think I will show a vacuum freefall clip later this week, to reinforce the result. Both times, the demonstration steered the discussion in the relevant direction.


6. We managed to spend a few minutes getting ready for the next lab, in which we will look at how surface area and mass affect the frictional force. We are taping force gauges onto the tops of ramblers — it is already looking promising.

modphys 32/180: Atomic Model

On a superficial level, atomic scattering of alpha particles is a simple process to understand. My students have usually been able to repeat back the main observations and conclusions: most alphas pass straight through but few are significantly deflected, so atoms are mostly empty space, with a small, hard nucleus. However, I have always suspected that my students didn’t really understand how the experiment worked theoretically, nor why this complex system was required.

Last year, I began to use a simulation in which I hide an object under a board and have students try to figure out what it is by shooting marbles at it.


I use blocks to hold up the edges or the board. It is good for these to stick out a bit so the students don’t conflate their effect with that from the hidden object. For objects, I have tried a number of things. A water balloon or barely-unflated rubber balloon provides a good elastic collision. A heavy round mass is good too, but produces a distinctive tink, which sort of ruins the mystery. One good way to get an inelastic collision is to increase the rolling friction in a region by putting down a piece of thin felt or newspaper. The former represents the Rutherford atom, while the latter is more of a plum pudding model.

It is hard to assess the effect of an activity like this, but it seems to me that my students have been able to talk more cogently about the Geiger-Marsden experiment when they do the hidden object activity first.

A problem we run into is the direction the marble flies out of our clumsy hands. I think I will try to make some launchers, similar to what you would find in a pinball machine. Such launchers could also be good for studying projectile motion.

#modphys 25/180: Water Tower

Adjacent to the (beautiful) apple orchard behind our school is a disused water tower. A conveniently-located platform is approximately 25 m above the ground. I was able to obtain permission to access the platform.


I’m keen to use the tower for several classes. In math, we’ll make some angular measurements to determine the height of the platform. For my physicists working on the Constant Acceleration Particle Model, we’ll drop some objects and measure the acceleration of gravity.

Do you have any ideas I could try?