Using video to capture quadratic motion

In my MBF3C class today we observed quadratic motion and modelled it with, and online graphing calculator. I’ve recreated the steps here (with some fake data).

First, I went to Desmos and projected a blank Cartesian coordinate system onto the screen at the front of the classroom.

Then, I found a black rubber stopper (I teach in a science classroom) a little more than an inch across.

I asked for two volunteers who would be willing to throw things at each other. That was easy.

They practiced lobbing the stopper back and forth to each other in front of the screen, trying to get the black rubber to crest visibly near the top of the grid. Eventually they were confident they could do it.
I held my iPhone in landscape orientation and recorded a slow-motion video. After a few attempts I felt we had a successful toss, and the students returned to their seats without injury.

We scrubbed through the video slowly and recorded the x– and y-coordinates for each major tick of the x-axis.

Then we plotted the points in Desmos (using the Table feature):


We then graphed a generic quadratic using the vertex form and its parameters (y=a(x-h)^2+k). Desmos provided the sliders for each parameter:


As a group the students decided to make a negative and small, to flatten the curve, then they adjusted h and k to fit:

Some interesting stuff to note about the process:

  • Even using 120fps there were places near the edges of the curve for which it was hard to see the coordinates (blurring and gaps).
  • The vertex wasn’t on the y-axis, which was surprising to the students.
  • The glare of the projector made the grid a little hard to see.
  • We had to have the lights out to make the grid visible at all on camera, and the dim lighting made the video a bit grainy.
  • The parabola we fit to the data worked really, really well.

What’s next

I’m going to perform some more motion tasks with them to get more quadratic data, and we’re going to do some curve-fitting to model and predict things (for example, how far can you throw a ball off a 10th-story roof?).

I’d like to have a large, physical grid on the wall or something so that I can have the lights on when we record video.

I want students to record video and analyse it. Lots of them have iPhones, and I bet some of the other phones can take good, crisp video. If not, there’s some learning there too (about interpolation if nothing else).

I’ll try some other phenomena also.

A video you can use

Here’s another video we took, if you want to use it:


4 thoughts on “Using video to capture quadratic motion

  1. If you’re allowed, you could put glow-in-the-dark tape on the wall to make a grid. Or maybe the art or tech departments have a couple of large pieces of cardboard you could set up?

    Or maybe you could “do it in post” in iMovie or something—get a transparent image of a grid, and superimpose the image over the video? (I’m not familiar with Apple devices, but I assume it should be possible somehow.)

    This is a neat idea, though. I hope I’ll get the chance to try something like it one day!

  2. I thought about the post-processing, but I want something that the students can do on their own devices. I might try to find some skinny dark tape to mark out a grid. If I do, I’ll post pics!

Leave a Reply

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

You are commenting using your 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 )

Google+ photo

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

Connecting to %s