In order to measure the horizontal velocity of the the ball, we need to have distance over time. We'll get the time portion based on the number of frames in the video (30 frames per second). To make sure we get good measurements, I had students place masking tape perpendicular to their ramp at a set interval. We would measure the speed of the ball over that distance.
Plug in the phone into the computer. Download the video file. Now your ready to go. There are two different ways to find a speed using the Tracker software. First, the easy way. The Tracker software advances video files one frame at a time. Because standard video is 30 frames per second, this gives us a rate we can use for our calculations. Find the number of frames it takes to travel the set distance between the pieces of masking tape then divide by 30 to get your horizontal velocity: distance/#frames * 30frames/1second = distance/#seconds.
The second way actually uses the tracking portion of the software--hence Physics "Tracker". It's pretty neat but takes longer to explain. If your interested, here's the link.
Students measured off from the end of the table (not the base) and placed the can on the ground. The air was tense; grades were riding on the accuracy of their calculations and measurements. It turns out no one needed to worry.