Trajectory

I am critiquing myself again. This time I want to examine an early attempt to create with my students an elaborate digital document about the physics of a trajectory.

A trajectory is the two-dimensional path an object takes when moving through space under the influence of gravity. It's the path of a thrown ball, for instance. There are two aspects of trajectories that students have difficulty understanding. One is that gravity (at the Earth's surface) is always directed downward, that it is constant, and that it is constantly pulling down on the object regardless of how the object is moving. If the object is moving up, gravity slows it down. If the object is moving down, gravity speeds it up. The other aspect is that gravity does not work in the horizontal dimension. Whatever horizontal speed the object has remains constant (ignoring friction).

The velocity of an object can be represented with an arrow, called a vector. The arrow indicates direction, and the length of the arrow indicates speed. Any arrow can be "broken up" into a vertical part and a horizontal part, and this analysing of vectors allows one to understand the shape and size of the trajectory. It's so much easier to show this with a diagram or an animation.

There are several layers of story here. First, a trajectory is the "story" of an object moving through space. Using vectors, this "story" can be separated into a vertical and a horizontal "story." The video also tells the story of my students launching an object and watching it fly. The various diagrams superimposed on the video tell a story of how the overall animation was constructed.

I want to look at this last story in detail, because I believe it can be examined as a remix practice, and I want to describe the tools and techniques used because they are unique to physics education. I referred to Lankshear & Knobel's appendix to Chapter 4 in New Literacies about remix practices to examine this animation.

The students are standing in a hallway of the school outside the classroom, and are using an air pump to launch an empty plastic water bottle into the air. The resulting trajectory was recorded on a Surface tablet by a student who had to resist the urge to follow the bottle as it flew. The students launching the bottle had to ensure that most of the trajectory appeared in the frame of the camera. All this took some practice.

Once a good video was recorded and uploaded into the school's network, the video was opened with a tool called Tracker. Tracker allowed students to isolate and slow down a section of the video, and then locate the bottle frame by frame, thus marking out a trajectory. This path could be edited and superimposed on the video (in a manner resembling rotoscoping). This work was done collaboratively on a Smart Board. This is the yellow trajectory seen throughout the animation.

At 0:20 yellow velocity vectors appear. These were automatically generated by Tracker based on the trajectory data. At 0:37 bold yellow vectors appear. These were hand-drawn by me on the Smart Board using Smart Notebook.* I then invited students to draw by hand the vertical (green) and horizontal (blue) components of the bold yellow vectors, which appear at 0:46. Note that the vertical components change, and the horizontal components remain constant. These component colors were consistent with notes and other diagrams used earlier in this physics unit.

I later arranged Tracker and Smart Notebook so the various pieces would be aligned and layered. I used CamStudio to record a screencast as I manipulated the programs, causing graphic elements to fade in and out, resulting in the final animation. The animation formed the basis for further study of trajectories. A lovely addition that could be made would be an audio narration, which could be scripted and read by the students.

It was the layering and manipulating of the various visual elements that constituted remixing. As with any kind of digital remixing, success relied on a certain expertise coordinating several pieces of software. The students also needed an understanding of vectors, and how to properly handle the equipment out in the hallway.


* A 90-day free trial of Smart Notebook is available. You don't need a Smart Board to use it, but if you have access to a Smart Board, you can use its serial number to register your copy of Smart Notebook so it won't expire.