How I Teach… Work & Energy

Interestingly enough the work and energy unit/chapter has become my litmus test for whether or not I’m going to invest time in a resource. It was what spurred my frustration with The Physics Girl’s AP review series (although I’ve learned that when you’re actually commissioned by someone like PBS you have to bend to the whims of the corporation).

So what’s the litmus test? Open the resource to the first page of the Work & Energy chapter. If you see “work is defined as force times distance” close it and move along! First of all, let me be totally clear, that was me early in my career. I taught that work was the dot product of force and distance, we did a lot of different calculations and then we defined energy and did conservation of energy. My frustrations began with the fact that students were not transferring the idea of work over to conservation of energy. They deepened when upon reflection I realized my angst was because the core idea is not just that “energy is conserved” but that work causes a change of energy in a system.

Enter the new dynamics. I’ve talked about how the structure of your units is really going to guide students to what is important. If you start the lesson with an equation, then they are going to assume that equations are what’s most important. However, if we start with their preconceptions, build on that knowledge and form models we can get a little farther than equation hunting.

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This is part of a series!
Part 1 (Work) Part 2 (energy bar charts) Part 3 (problem solving) Part 4 (Lab)

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I start by asking students to name types of work. The list looks something like this:

• Homework
• Housework
• Yardwork
• work work (a job)
• Wood working

and so on…

Then I ask students what is shared amongst all of those ideas. I’m looking for two answers, that they all require effort and that they all end in a change: Do your homework and your brain grows, work a job and you get paid… and so on.

So then I give students a list of tasks: lifting your backpack, holding your backpack, dropping your backpack, walking with your backpack (at a constant speed), climbing the stairs with your backpack, and I ask students which of the following are an example of doing work. We don’t share answers quite yet because I don’t want to participate in “expose and shame” where we trick students into marking the wrong answer. After they come up with this list then we formally discuss work as “a change in energy of a system due to the application of forces”. I emphasize the change which is in alignment with our original definition and “application of forces” which is the “effort” part they mentioned earlier. We go back through the examples and have a discussion about which are work and how.

Note: We’ve already discussed systems when we did forces, so there is a review of this idea as well.. the concept of systems is critical to student understanding of work and energy so if you’ve not done systems yet you need to hit this hard!

I’m going to include a few of the sample problems we work together in class to hit different ideas:

I ask this question right after our intro to work. I let students come up with lots of ways to reason the answer. The “correct” answer is that the force is perpendicular to the displacement, but this is also a good time to discuss that a “before” and “after” snapshot would also look identical, or that with each orbit the displacement is zero!

I ask this question in two ways: first as presented, then I ask them how the ranking changes if they were asked about the work done on the OBJECT. This is also a good place to discuss what negative means in the sense of work (positive work ADDS to the system while negative work takes away from the system)

Also of note: in AP I tell them that any time they get a graph they should ask themselves “does the slope tell me anything, does the area tell me anything” slope is essentially dividing the properties while area is multiplying (I know this is a major oversimplification, but it’s an algebra based course). I show them a graph of force vs displacement and ask how they find the work done (area!) they have a few practice items with these.

We run the spring lab where students discover Hooke’s Law and then I ask them to determine the amount of work done on the spring. Most students are able to get to the idea that its 1/2kx^2, but I do always have a few groups that want to just sub in kx for force and end up omitting the 1/2. This is a great conversation to have in a board meeting.

In my regular classes I run this great desmos activity I found by another teacher (try it out here).

First, students move the sliders to make the graph match the scenario…

Then that graph is reproduced on the next slide so students can use it to perform the calculation

This familiarizes students with graphical representations and the idea of how positive and negative work affect the system. I added one last slide to the original asking students to review what they learned in the activity and predict the work done by the spring in their lab

When we are ready to move to energy, I open with the following question:

A ball is dropped from rest.

1. Define the system to be just the ball. Sketch a diagram showing the ball and the earth and identify the system by drawing a dashed circle around the objects in the system. Include any relevant forces. Is work being done on the ball? Explain your answer.
2. Define the system to be the ball and the earth. Sketch a diagram showing the ball and the earth and identify the system by drawing a dashed circle around the objects in the system. Include any relevant forces. Is work being done on the ball? Explain your answer.

This is a great way to talk about how the same situation can describe work or not. The gravitational force is clearly inside of the system in #2 and therefore is a NON example of work.

We’ll discuss how I move from work to energy another day!