Measuring velocity of objects using video clips

This material is replicated on a number of sites as part of the SERC Pedagogic Service Project
Initial Publication Date: September 20, 2010

Summary

Usually we ask students to solve word problems where they are given all the information they need to solve the problem: "A car moves with velocity of 20 m/s..." In this activity, students learn to extract data about the motion of objects using videos. Several examples of videos are provided, as well as instructions for using some of these videos to measure velocity. Once this technique is mastered, students can begin to use the velocities of objects in the videos to explore other aspects of mechanics, such as collisions, conservation of energy, or projectile motion.

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.

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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:
Instructions for determining velocity from a video clip are in this file: Student instructions for measuring velocity using video clips (Microsoft Word 180kB Sep6 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.

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.