Measuring velocity of objects using video clips
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Initial Publication Date: September 20, 2010
Summary
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If the video is taken carefully, students do not need video analysis software, just a simple video playback software, such as Quicktime player, that allows students to advance the video one frame at a time.
Learning Goals
Students will learn to analyze video clips an extract data about the velocity of moving objects. This will reinforce the concept of average velocity. In addition, this activity sets the stage for other activities that build on this techniques, such as the Conservation of energy while rolling down a hill and the Conservation of energy in a roller coaster activities.
Context for Use
Intended for use in an introductory physics course covering mechanics. This activity requires students to have access to computers, ideally one computer per student. The computers should be equipped with a Quicktime player, or other video player that allows students to view .mov files frame-by-frame.
Description and Teaching Materials
Several video clips are included. These should be downloaded so they can be viewed frame-by-frame in the classroom:
- girl rolling down driveway on scooter (Quicktime Video 216kB Aug29 10)
- students on giant swing ride 210 frames per second (Quicktime Video 9.3MB Sep19 10) Students on giant swing ride at an amusement park, filmed at 210 frames per second
- tennis ball falling into water, filmed at 420 fps with grid (Quicktime Video 2.1MB Sep19 10)
- low friction toy ("hover puck") sliding across floor (Quicktime Video 1.4MB Aug29 10)
Teaching Notes and Tips
This activity should be introduced when discussing the definition of velocity as the displacement divided by elapsed time. With this activity, address these two quantities separately. Students determine the elapsed time by counting the number of frames for an object to travel its own length. For example, they count the number of frames from when the object enters the field of view until the object is completely in view. They use dimensional analysis to convert this number of frames to a time by using the frame-rate.
Students struggle a bit a first recognizing that the distance that the object moves during this interval is the length of the object, rather than some other known distance.
Note that once students have mastered this technique, there are many possibilities for extensions. For example, the tennis ball video is an ideal introduction to free fall, with the tennis ball in free fall on the way down and the water droplets in free fall on the way up! In addition, it is interesting to compare the velocity of the ball as it hits the water with the velocity of the drops of water as they emerge from the splash. Similarly, the hoverpuck video is an introduction to motion in 2-dimensions, but the object is traveling at a constant velocity in both the x- and y-directions.
See a complete description of how to teach with video clips, including pedagogic strategies and a collection of sample videos.
Students struggle a bit a first recognizing that the distance that the object moves during this interval is the length of the object, rather than some other known distance.
Note that once students have mastered this technique, there are many possibilities for extensions. For example, the tennis ball video is an ideal introduction to free fall, with the tennis ball in free fall on the way down and the water droplets in free fall on the way up! In addition, it is interesting to compare the velocity of the ball as it hits the water with the velocity of the drops of water as they emerge from the splash. Similarly, the hoverpuck video is an introduction to motion in 2-dimensions, but the object is traveling at a constant velocity in both the x- and y-directions.
See a complete description of how to teach with video clips, including pedagogic strategies and a collection of sample videos.
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Assessment
The ideal assessment is to give students a new video clip that they have never seen before and asked to determine the velocity of the object.