Direct Measurement Videos > Activities > Keep in Time - Student Activity

Keep in Time - Student Activity

Matt Vonk, University of Wisconsin River Falls
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This activity is based on the "Keep in Time" video and allows students to measure the speed of sound in air in a way that is intuitive and visual. I use this activity so that my students can practice making velocity calculations. A quick and easy way for students to measure the velocity of sound is to look only at the first and last claps, but the video can also be a great way to practice graphing skills and graph analysis skills.

Learning Goals

Students will:

  • Describe why the clappers don't all clap at the same time
  • Calculate the speed of sound in air
  • (optional) Make a graph of distance vs. time of the claps
  • (optional) Relate the slope of the graph to the speed of sound
  • (optional) Find meaning in the y-intercept of the graph

Context for Use

This activity assumes that students know how to calculate velocity as a displacement over a time, but other than that it doesn't assume prior knowledge. If the graphing method is used then students may require additional guidance and support related to graphing. If students measure the velocity by only looking at the first and last students clapping then the activity takes about 20 minutes. The graphing method takes about an hour.

Description and Teaching Materials

Video file: Keep in time
Student video library - allows access to all videos for students, without links to instructor materials.

Here is a worksheet (Microsoft Word 2007 (.docx) 2.5MB Aug24 14) to guide students through this activity and worksheet solutions (Microsoft Word 2007 (.docx) 3MB Aug24 14) for instructors.

These instructions have medium scaffolding, giving students fairly detailed instructions, but requiring students to determine the best way to measure the speed of sound in air. An instructor may chose to provide less detailed instructions to encourage independent thinking and problem solving.

Teaching Notes and Tips

Students who watch this video sometimes think that the students in the video were waiting until they heard the metronome beat to react, but they weren't, they were clapping together with the periodic beats they heard.

If you have the time, I would encourage you to have your students try the graphing method. The results turn out closer to the expected value, it makes sense to use the data from all of the students (not just the first and the last one), and I think that the particular way that the data deviate from a straight line is especially instructive.


There are many other videos in the "How Fast is that?" series that would be great for assessing student proficiency at measuring displacement, elapsed time, and average velocity. If the graphing solution is used, then students could also be assessed on their ability to make and analyze data graphically.

References and Resources

Here is a link to the Hyperphysics page on the speed of sound in air. It has more detailed information about how the speed of sound depends on temperature.