Why Direct Measurement Videos are Awesome

Direct Measurement Videos?

Yes, Direct Measurement Videos. I attended a workshop at the 2014 AAPT Summer Meeting by Peter Bohacek (@bohacekp) and Matt Vonk (@matt_vonk), and throughout the 4 hours, all I could think about was “Ahh! The possibilities!” I can’t wait to implement them this school year.

The “Direct Measurement” part comes in through the overlays that allow students to make measurements directly from the video.



This one is one of the simpler videos in which students are asked to calculate the velocity of the ping-pong ball. At first glance, this may seem trivial. Just  do distance divided by time, which you get by using the frames and frames-per-second values, and bam. You’re done. But just because the computation seems trivial doesn’t mean the physics are!

Why I love them

What would otherwise be a mostly trivial formula-hunting textbook problem has become a wealth of enriching discussion. For instance, students will need to decide how to measure the distance the ping-pong ball travels. Should they measure from the front of the ball? The back? The middle? Does it matter? How do we get the time? What’s this frames-per-second thing?

Another great feature is that their customized video player allows for frame-by-frame playback, thus opening further discussion. What frame should we start the measurement? End it? Why? There are lots of opportunities for students to justify all the choices they make along with many different paths to getting a solution. Having so many different ways of solving the problem, I think, goes a long way in helping students see that models and assumptions are made about a physical system based on intuitive and logical choices. And those choices need to be justified in order for any calculations to hold any weight. I think that this will help students move away from the idea that all these crazy equations are mystical, immutable descriptors of objective reality.

One activity we did today fit hand-in-hand with another idea that I’m in love with that I got from @kellyoshea: goal-less problems. The basic idea is that instead of giving students a situation and asking them to solve for various parameters, students are just given a description of a situation. They’re asked to figure out which models apply to the situation and to figure out, based on the given information, what things they could solve for.

We ran through the activity for a video of a toy plane flying in a conical circle.


What we did is break into groups of 2-3, and each group came up with a problem. Many of them were fairly obvious (to us physics teachers, that is) things to ask: How long is the string? What is the radius of the circle the plane flies in? What’s the plane’s angular velocity? But there were also ones like “if the plane’s speed were to double, what would the scale read?” or, my favorite, “how long is the propeller? Wow. Peter read off some of the things his students have come up with, and they had just as many interesting and unique problems as we did, many that none of us came up with.

We then traded problems with another group, and tried to solve their problem. The overall goal was to come up with a problem that would stump another group in a sort of “face-off”, but the group had to be able to solve the problem they wrote. This encourages students to think about physics and physics problems in an entirely new way. Being able to write a good physics problem requires a thorough understanding of the physics involved.

What else?

 I was especially excited about the last video we looked at.


The screenshot above isn’t of the exact video we looked at, but it’s similar. The video we looked at is still in the prototype phase, so it’s not ready to be released.

The bulb on the left and Joly Photometer are the same. However, the photometer is attached to a bunch of smaller light bulbs at a fixed distance away, and the bunch-of-smaller-light-bulbs-and-photometer system all move together on a track. There were 9 videos in total, each one turning on one additional smaller light bulb. If you plot the distance from the bulb on the left of the smaller-light-bulbs-photometer vs. the number of bulbs lit at the point where both sides of the Joly Photometer are equally bright, you get an inverse-square relationship for distance vs. intensity. My first thought?

Paradigm lab!

I’m not confident in my ability to set this up in a way that students could use to get meaningful measurements. I’m not sure that I want to devote the time and resources to building a reliable setup. Instead, I can check out my department’s laptop carts, and students can take all the measurements themselves. For labs with difficult and finicky setups and equipment like this? This is perfect.

I could rant more and more about how awesome these videos are, but I’m exhausted after my first day at my first AAPT Summer Meeting. I encourage you to check these videos out! I can see them being useful in any style of classroom, and they’re a much better alternative to spending a day solving textbook/worksheet problems when you can.


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