Teaching Methods

# How I Teach… Energy Part 4 – Lab!

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

I have this lab I received from a colleague, it’s an iteration of a lab I’ve seen in other places. Basically an object goes down a ramp, gets caught by a paper catch/index card etc and students are looking for some iteration of work and energy.

In the version I have students are asked to find a relationship between height and distance. The cool thing about this is it ends up that height is directly proportional to distance and related by the coefficient of kinetic friction alone.

Student’s work looks like this:

Students are asked to complete the lab with a hot wheel car and then again with a small mass attached to the car. To students’ surprise the lines are not identical. This really bothers students until we discuss what we were actually looking for. See, the lines are still parallel, but the car with more mass is going to have a greater momentum at the bottom and will require a greater impulse to stop. It’s a fantastic conversation piece.

I really enjoy this lab because it requires students to consider a new problem and then apply that knowledge to a lab setting. Research has shown that students don’t really learn content in the lab, they learn lab skills. I was always a little frustrated with the disconnect between all of the work students put into the theory and then the lab results themselves. So this time I changed things up.

Instead of giving students the lab hand out and letting them work in groups, when students walked into the room they were put into visibly random groups. Visibly random grouping just means you create the random groups in front of students so they see it was truly random. I’ve been immersed in the book Building Thinking Classrooms and the research on this is really cool.

Once students are in their groups and at a white board that is vertically mounted, I’m in the middle of the room at a lab table with the lab set-up. I verbally explain the set up and that I want them to derive a mathematical model for the relationship between height and distance.

Vertical whiteboarding is really cool and has several advantages. First, students are standing which puts them into a more active position, this gets more of them working. Second, it’s really easy to just look around and snag ideas from other classmates. Third, since they’re already standing it’s really easy to move around the room and discuss with other groups. The first time I did this what astounded me was the sheer number of students talking. Instead of it being maybe 4 or 5 leaders it was nearly everyone in the room! There was so much collaboration and ownership of learning it was magical.

So I did this with the first part of the lab. Next, I asked them to sketch what the graph will look like with the two lines. Almost all of the students sketched the two lines on top of each other. I want them to have the experience of their data not aligning with their previous ideas and having to reconsider, so we left it at that. Then students were off.

I’m going to finish this lab this week, so I’ll have to come back to update this post, but I love this activity and vertical whiteboarding gets a 10/10 every time.

# Day 2: Thinking about Relationships

Day 1 I run a HUGE physics smorgy: 11-15 demos/lab set ups with minimal directions. Students are told to play, investigate, explore, PAY ATTENTION and ask lots of questions. This is my hook into the class for the year. I’m able to observe the students, act ridiculous and ease the MASSIVE anxiety they walk into this class with.

The next four days we actually spend working with data and relationships. Specifically to build the skills necessary to analyze data on a graph and straighten it when needed. I have a reading I ask students to do ahead of time and then we go through the straightening process. These brilliant students (half of whom are in AP Calc) are completely flabbergasted by the straightening process. It just doesn’t. make. sense to them.

I decided to try something different today on the fly, and it brought about some great conversations. First I put up blank sketches of graphs depicting a linear, squared, inverse and square root function. I asked them to put the graphs on their white boards and write the relationships. The answers consisted of the following:

• “linear, squared, inverse and square root”
• y=x, y=x^2 (etc)
• y∝x y∝x^2 (etc)

This kicked off some great conversations. Are we in agreement, generally, about which is which? (yes). Are the equations really representative of the sketches? (We don’t know, there are no labels or numbers on the axes)

Next, I gave students four statements

1. “Momentum is proportional to velocity”
2. “A spring loaded gun is fired upward. The height of the bullet is proportional to the compression squared”
3. “Velocity is inversely proportional to mass”
4. “The period squared is proportional to the length of a simple pendulum”

I asked them to label the axes of their graphs with the physical quantities to match the statements. Here’s where the fun began. Students took a lot longer than I had originally anticipated completing this task. Here were the great conversations to be had:

• In science, we usually put the independent and dependent variables on the x and y axis. With these statements, is it obvious which is which?
• Since it’s not obvious, are answers where the axis are flipped wrong? (Not if they picked the appropriate shape!)
• So, we often are going to use slope to talk about relationships. Like, say, if we plotted distance on the y and time on the x what would we get? (speed…minds are blown)  The cool thing is if you plot the graph “wrong” you can look at the units,  and decide if they need to flip because you’d have seconds per meter or something. The important thing is whatever you tell me the relationship is, needs to match your graph.
• Then, of course, I let them in on the secret: we always list the y thing first. Literally all we are doing in these sentences is taking the math proportions, like y∝x^2 and saying, instead, height ∝ compression^2. It’s like the hugest lightbulb moment for students ever.

Now that they have that substitution thing in their brain, explaining how to straighten graphs is a snap. I was really pleased with the lack of frustrated and confused faces. Last year, I sadly, lost several kids during this unit. I wanted to cry so hard because we hadn’t even started physics and seriously questioned my lesson plans.

Tomorrow they finish their pendulum labs, so we’ll see how this all goes.

Meanwhile, AP Physics C is dabbling in computational physics for kinematics. More on that later.

# Pass Along – Modeling Waves

The pass along activity is one I developed shortly after attending a Kelly OShea workshop. I wanted to combine modeling with the strengths of white board speed dating and board walks. At the time I didn’t have the large whiteboards and for this particular activity I decided a piece of paper would work best.

Part I: I ask students to draw in a pictorial representation of what a longitudinal and a transverse wave might look like.

Students are then told to pass along their paper. I predetermine groups randomly for this activity. Three is best, but if I don’t have a factor of 3 then I put the stragglers into groups of 4. It looks like this:

Student 1 -> Student 2 -> Student 3 -> Student 1

Part II: After students have passed along, they are required to look at the work done by their peer and explain, in words, why that person drew what they drew. Much like speed dating, this requires each of the students to get in the minds of their peers, but without the opportunity for their peers to explain.

Students then pass along again.

The third person takes a look at the previous two answers and then has to think of a way to model each wave type with their bodies.

After the three pass alongs, students get into groups, at this point each paper has been touched by the same persons. They discuss their answers and then they have to get up in front of the class and model with their bodies each wave type.

The physical modeling is great in that the kids are up and moving, but it also provides an opportunity to have a discussion about the model. 7th hour we had a discussion about whether or not doing the worm accurately models a wave (nope, the particle is moving across the room). Similarly, I had a few groups move their whole line down the room which brought up the discussion point about what a wave transfers and doesn’t transfer.

Afterwards, we will go out as a whole class and model transverse and longitudinal waves using an 8-step count.

# A Spin on Energy

Last week I ran a pretty straightforward lab:

1. Put 120cm of hot wheel track into a design of your choosing
2. Run a ball down the track
3. Record velocity with a photogate
4. Repeat at 10-12 locations
5. Plot the energy curves.
6. Plot Translational vs Rotational Kinetic energies and find the rotational inertia constant.

Students should see a transfer of kinetic and potential energy which makes sense. Of course, students should also expect to see a decreasing total energy curve because of friction constantly taking energy from the system.

I had two fun surprises I got to incorporate:

1. The shape of the TME curve

Inevitably this curve had a particularly sharp drop off at one moment in time. I had students sketch their tracks on their whiteboards in addition to their lab results. Do you notice anything? The largest drop off in TME corresponds to the moment where the ball is at the bottom of the hill. This serves as a great review of work and circular motion. Frictional force, as we know, is dependant on normal force. The normal force of the track changes and corresponds with its shape. We can actually predict the drop-offs in TME based on shape and even determine the work done by friction.

1. A group with “bad” data.

Their data wasn’t actually bad, they obviously had forgotten something when they set up their formulas in the spreadsheet. But was there a way to find this without redoing the whole data spread? Absolutely. After creating a large circle to share whiteboards, we honed in on the group where the TME curve was mirroring the potential energy curve. The rest of the data seemed good…there was an obvious trade-off of PE and KE…although the curves weren’t as high as they should have been. So what was the problem? I selected a student to draw in where the energy curve should be, based on the shape of their track and everyone else’s data. She drew in the curve. Next, I asked students to note where this curve was and where the PE curve began. It was at 0.3 J with PE starting at 0.6 J Then I asked them to note where the KE curves were at… they were at 0.03 J. Notice anything??? They were off by a factor a 10! Where could a factor of 10 be? Did they forget a 9.8? Did they convert grams to kilograms properly? cm to m? Upon examination of their equations, they found the missing 10 and…TA-DA! Fantastic results.

I think it’s really important to note the value of both exercises. The lab itself was relatively simplistic, but it lent itself to fairly complex conversations.  I think this is especially true for the group with the “bad” results. How often do our students present with this and either (1) Default to “well my data must be bad” or (2) Start from scratch, rather than locating the mistake? In this way, students were able to critically analyze, strategize and problem-solve. It turned out to be a really easy fix.

Oh and the slope of the translational vs rotational KE? Yea that came out to 2/5….exactly. That’s super exciting!

Teaching Methods

I visited my alma mater today. The entirety of Green Street on campus is closed to traffic due to all of the construction. Buildings have gone down and come up and I half expected time to still be frozen in the year 1967 in the physics building.

When I walked in I found quite the opposite. Not only newly renovated rooms, but there is actually a women’s bathroom on the fourth floor. (This was always a running joke)

The reason I spent 6 hours in my car today, however, was to visit the Physics 101 class. My former adviser, Mats Selen, has been working on a new project: the iOLab. The concept is simple, it’s a multisensor system in a box. And it can do everything your \$10,000 of Vernier equipment can do… for a little over \$100. It connects wirelessly to your computer and runs with free, opensource software that does all of the analysis our expensive programs run.

On the other side of the coin, however, is a radical change in how the introductory level classes are being taught. When students walked into the lab, they had done a pre-lab experiment earlier…..at home…..with their iOLabs. Quite simply, they made a stack of books, put another book on top by its edge and then looked to see how the force changed with the iOLab as it was placed at different distances from the book stack. Data were submitted ahead of time for credit. Students discussed the results at the beginning of the lab and then were given their task. It’s the classic peg-board demo, however, students had to find a way to relate the force to the placement of the probe if the pivot was located in the top corner.

This was the sum total of the direction given to students.

Within about 20 minutes all students were taking measurements. Some were looking only horizontally, others were looking both horizontally and vertically. Questions arose about the approach: if we change the angle at which we hold the probe the force will change. Are we supposed to do this with a horiztontal force too? I think that’s impossible.

They were told it’d be great if they came up with a mathematical relationship, but they’re just looking for the trends.

Within an hour students were plotting their data, recognizing it was an inverse relationship and running the curve.

One group really wanted to get the formula.

Another group recognized the torques should be equal and started calculating all of the torques. Percent uncertainty was one of the objectives focused on, so I wanted to see how well they were grasping that concept. I looked at the torques and noticed the values were .14, .14, .14, .15, .16. So I asked them how they were going to decide that those were constant and not increasing. They responded that they would have to determine their percent uncertainty and compare what was acceptable to those values.

Now, clearly there are major differences between high school junior and seniors and pre-med juniors and seniors, but at the same time, it was still remarkable how they were approaching the lab, developing their experiment and writing up their labs. It is something that very much excites me about the potential use in the high school classroom (and online classrooms, and college classrooms etc)

I also asked students about their previous physics experiences. About half reported they had taken physics in high school, ranging from regular level to AP Physics 1. ALL students reported that they felt they had a FAR BETTER grasp of physics now in this course, compared to their high school course. Several students who said this felt the need to insist they still had a great high school teacher 🙂

The message, however, is clear: we need to give our students the opportunity to design and evaluate their experiments.

Also, the iOLab is a very exciting new piece of equipment. Morten Lundsgaard, currently the Coordinator of Physics Teacher Development
Instructor, is hoping to run workshops and/or a camp for high school teachers. If you are interested you should contact him!

# Slicing a Cylinder for Moment of Inertia Integration

Guys….we’re in the throws of rotation. And at least one of my poor students has calculus immediately preceding AP Physics C. I feel so bad for her. The day we started she had made up a calc quiz, came to day 1 of rotational inertia, then went to calculus. Oh did I feel her pain.

Arguably the most difficult part of deriving rotational inertia is the visualization of how to go about the integration. I mean, let’s be honest, once we find how to express dm the integration is always an easy one.

Part of the problem is getting students to understand what it means to say things like dm, dV, dA, etc. They understand the definition linguistically, but it’s really hard to think of it practically. Tell them that dr^2 is zero and their minds are blown and bothered.

Day 1 of cylinders did not go well. Arguably, in part, because we were short on time. But also because the what why how was overwhelming.

I remembered a demo someone had shown where they 3D printed their objects to roll down the incline. They had actually made nesting cylinders, which then served as a great way to discuss integration.

I’m trying to think of a way to visualize each of the d-steps of the cylinder integration for my students with materials I have on hand. As I’m digging through the closet I notice the slinky coil. It’s nearly perfect!!!

Ideally, I wish I had one with nice thick coils so we could take about the cylinder with R1 and R2, but this will suffice for the most challenging part.

So imagine you have a cylinder of length L, and inner radius R1 and outer radius R2 and would like to determine the moment of inertia about its center…

First, as always let’s define rho, but we have to find dm in terms of r. So how do we do that?

Well, let’s take some horizontal slices, where each slice is dm… now we can see that dm = rho*dV…but wait… what is dV?

Well, if we make those slices infinitely small…is there really a volume left?

Ah! so dV is really dA, and we are looking at it across the length of the slinky, so dm = dA*L!

Conveniently, I know that A=pi*r^2, so dA = 2*pi*r dr

And the rest is substitution!

Teaching Methods

# Modeling vs Intentional Modeling

“I use modeling, do you?”
“Uh…no, but I’m interested in learning about it”

I felt like such a noob when I had this conversation a few months ago because literally, everyone else at my group seemed to be doing this already. I was at a workshop on whiteboarding after a talk on standards-based grading and modeling and I thought, “wow, she really has it together… I have a LOT of work to do” (Does anyone else have this overwhelming feeling of inadequacy in the classroom all. the. time. or is it just the mom-guilt extended into the classroom?)

So I have started incorporating some things here and there as I’ve gone along, and I recently looked into Etkina’s resources (I started using parts of her book last year). As I poured over Etkina’s labs and our workshop speaker’s resources I realized: I HAVE BEEN DOING MODELING ALL ALONG! Mostly because it’s just the way I already think about problems. It just didn’t have a fancy name, and more importantly, I wasn’t always doing it intentionally as a teaching strategy.

I’ve decided that the intention is really the key in modeling as a teaching strategy. I think good physicists are good at models but bad at teaching them. We do it so seamlessly in our own work we fail to realize that type of thinking is not seamless or natural to the general public.

Cue modeling curriculum

Models are just any representation we use for a situation: pictures, free body diagrams, motion diagrams, graphs, mathematics etc. We need to work our kids like gymnasts, very intentionally using and practicing these models so that our students become flexible and natural at using them on their own for any scenario.

This is the paradigm shift: teach the model first, and the physics as a result of the model. Too often physics teachers (especially physics teachers not trained in physics) teach all this physics stuff, then all these equations for particular problems and then maybe shove in some graphs at the end. The problem is that students fail to see the bigger picture and physics becomes a class where students are attempting to memorize a million procedure for a million different problems, rather than learning a handful of approaches and selecting the best one or two for the problem at hand. The clearest example of this in my current classroom is how I am teaching two-body problems. I have made a huge deal about the fact that all of the physics is in the FBD. Because learning the general process for FBDs is a lot easier than trying to memorize separate processes for ramps, Atwood machines, modified atwood’s and oops! Now there’s friction!

The next most important part of this is to teach students how to communicate with one another using their models, and this is where the value of whiteboarding comes into play. I believe very strongly in letting the kids move around the room to see whiteboards without having a board representative at each board. The reason for this is that the students begin to realize that it’s hard to make sense of what someone has done if you don’t provide enough detail. Students can then ask these questions and leave them at the board before we come together as a whole group for discussion.

I decided to use modeling very intentionally in the classic coffee-filter air resistance lab. The original lab I had snagged from someone had a bunch of background info and then asked students to skets the velocity and acceleration graphs. I got really tired of marking the same things on everyone’s papers last year and realized this year that this is a perfect opportunity for modeling.

When students walked in today their desks were in groups of four with a whiteboard. I asked them for the following

1. A free body diagram at t=0, sometime before terminal velocity, and at terminal velocity
2. Acceleration expressions for each of the diagrams
3. position, velocity and acceleration vs time graphs.

It was so cool to watch them work, discuss and argue. The FBD’s were relatively easy, the discussions mostly about whether or not to put air resistance on the t=0 diagram.

The discussions about the graphs were far more interesting. Many students were working with the graphs as unique units, rather than considering the relationships from one to the next. Inevitably we had piecewise acceleration graphs and linear acceleration graphs and linear piece-wise vs curved velocity graphs.

I asked the kids to cite similarities and ask questions about differences. One group today started changing their board before attention was drawn to them. It offered a fantastic opportunity to review the graph models and review the relationships.

One of my favorites was a group that decided the curve of the velocity graph was quadratic, so they started taking the antiderivative for the position function. They noticed the constant slope portion in many of the other graphs and asked the question about it. Then they realized (#overachievers) the velocity graph wasn’t really quadratic.

I realize this particular example isn’t quite model-based learning through and through as I did not allow them to experimentally discover the exponential function relationships, rather after discussing that all of these changes were continuous I gave them a brief taste of the calculus/diff eqs ending in “solution is always in the form….” and hey, doesn’t that look like the curve we agreed upon?

We only collected data today, so I’m really curious and excited for what their write-ups are going to look like Wednesday!

I’ll keep you posted 🙂