I Love Lab Practicums and So Should You

I love lab practicums. If you don’t use modeling instruction, the name may have a different meaning for you. In modeling, a lab practicum is a deployment activity in which students are asked to use the model they have developed to accomplish a specific task. Ideally, the arbiter of success in the task is Mother Nature. They might be graded, might not be, but they always provide students feedback on their understanding of the model being studied.

The classic example in modeling is the constant velocity buggy crash (aka the two-train problem) Having already determined the velocity of their own buggy while developing the constant velocity model, each group is paired with a second. They are challenged to use the constant velocity model to determine where the two buggies will collide if they are started at opposite ends of a horizontal ramp. They cannot run both carts together until they have made their prediction and are ready to test it. The excitement and enthusiasm students show for these activities is infectious and the assessment of their knowledge is raw and honest. They can’t hide from it. Either the cars collide where they predicted or they don’t.

As part of the #physicsmtg that I took part in, we brainstormed and shared alternate practicums that we could use for many of the modeling units. Here is a list of some of the ideas tossed around: Lab Practicum Ideas. This is only a quick list and some of the names may seem strange. If you’re curious about any of them that I don’t cover below, just leave a comment. We also looked at Practicums for Physics Teachers by Henry Ryan and John E. Barber.

Friday afternoon, we decided to setup and test two different practicums and a demo that many of us hadn’t seen. Here are some quick descriptions that should allow you to utilize these in your own classroom.

1. Two Ramp Race (for Constant Force Model)

Kelly O’Shea shared a practicum with us that she had originally learned about from Matt Greenwolfe.

Start with one ramp already set up at a 10° angle and the other at 5º. Tell the students that you will start the cart on the 10º ramp one meter from the bottom. They have to determine where to start the cart on the 5º ramp, so that the two cars reach the bottom at the same time. You might let them run each cart separately or require them to just use their models to make predictions, but the test of their knowledge comes at the moment they place both carts on the ramps and let them go.

I like this one for two reasons. First, it’s a natural continuation from the constant velocity (crashing buggies) and the constant acceleration (race down the ramp) practicums. Second, it’s clear cut and requires students to demonstrate understanding of the core principles of the constant force and constant acceleration models.

2. Simultaneous Collision (for Conservation of Momentum Model)

Mark Hammond set this one up, but I neglected to snap pics. In this practicum, two low-friction plunger carts are placed on a dynamics track centered between two bumpers. To minimize the mathematical difficulty and emphasize the physics concepts, choose a total distance between bumpers such that the total distance covered by the carts is a round number (e.g. 50 cm). That is the total distance between bumpers would be 50 cm plus the total length of both carts end to end. Deploy the plungers and the carts will strike the bumpers at the same time creating clearly audible simultaneous sounds.

At this point, provide masses that are equal to the mass of the cart, so that the students can double, triple, etc. the mass of either cart. Give them time to experiment with the setup and discover the pattern rather than directing this part of the process. Once they feel that they have determined a predictive pattern, test them one final time by making one of the carts have a mass of 1.5m. If they can’t accomplish this, send them back to the drawing board rather than moving them on to the final stage.

The challenge arises when you next provide the students with a rock and ask them to use the setup to determine its mass. They may use a balance to measure the mass of the carts, but not the mass of the rock. By drawing on the predictive model they have built, along with their knowledge of conservation of momentum, they should be able to determine the unknown mass.

I find momentum-related practicums to be difficult to make exciting without expensive ballistic pendula or car crashes. This one is easily affordable and challenges students to reverse the application of a model in the same way as the Matching the Beat pendulum practicum does. The scaffolding in it is well done and customizable to the class that you are teaching. If students need the practice, do each of the instances above. If your class is full of super-stars, drop all or nearly all of it. I think that I’d like more practicums that I do to have this scaffolding feature.

3. The Tin Foil Capacitor (for Electrostatics model)

Frank Noschese led us through his awesome tin foil capacitor demo and we discussed how to develop this into a practicum. Check out Frank’s post (linked above) for a video demo of the setup. This is a great demo that can be used to look at the distribution of charges on the surface of a conductor. Electrostatics is very light on practicums and I think that there is one lurking in here somewhere, particularly if you roll it up with an insulator sandwiched between the layers of the roll (thanks for the idea, Frank!). This could get at capacitance and how it depends on area. I need to think on this one more over the summer and try it myself.

When it comes to assessing with practicums, the teachers in the room had a wide variety of practices. For instance, Kelly and Mark use the Two Ramp Race as a test in their class. The setup is in one room and when the students are ready to try, they can enter and test their predictions. On the other side of the fence, I tend not to grade practicums and only provide verbal feedback about their process both during and after the activity. Of course, since adopting standards based grading, I’m thinking that these practicums would make ideal moments to score students on the relevant standards. It would provide one more data point for them to measure their progress.

So, modelers, what are your favorite lab practicums?

Edit: Due to my non-existent Latin skills.


  1. One idea that just came to mind from reading this blog: during Unit 7: Energy, after the “Energy Transfer Lab,” elevate the track opposite the spring launcher and have the student determine what force to apply to make the cart stop its forward motion at a certain location. This would bring together several models that have been developed up to that point, and reaffirm the conservation of energy model for that unit. We did a slightly different take on that during my workshop. I think making it a lab practicum might make more sense.

    1. Scott, thanks for using the singular form “practicum” correctly. I had a hard time reading this post, because I winced every time the plural “practica” was used as a singular noun.

      When converting Latin nouns to English, it is now acceptable to use a standard English plural form on the Latin singular, but not to use the Latin plural as a singular—”practicums” is now an acceptable plural, but “practica” is never an acceptable singular. It is the same for “curriculum” and “curricula”. It should be the same for “criterium” and “criteria”, but the plural “criteriums” is rarely used except in bike racing.

      On the modeling—are students expected to project vectors and use proper trigonometry to compute the forces on the 5-degree and 10-degree ramps, or are they expected to guess that one ramp provides about twice the horizontal acceleration as the other? Does increasing the angle (say to 45 degrees) completely mess up their models? At what point does the transition from the ramp to horizontal table mess up even reasonable models?

      1. Thanks for clearing up my Latin. I went back and forth pretty loosely with the usage as I wasn’t quite sure about the correct forms of the word and my (very) cursory research seemed to indicate that practica was acceptable. I’ll be sure to make the changes to the singular instances.

        I haven’t done the ramp practicum in my class yet. I would probably do this after the constant acceleration model but before the constant force model. In that case, I’d expect my students to use the CA model to develop the answer. So, I’d expect them to be able to show that the acceleration of the cars is dependent on the angle of the ramp and then relate that acceleration to the displacement, either algebraically or graphically. Of course, they may come up with another equally valid approach. I think that the model is fine if the ramp is tilted at 45 degrees, but the uncertainty in human reaction time would become pronounced with the high acceleration produced by such a steep incline.

        Thanks for taking the time to comment.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s