Direct Measurement Videos > Video Library > Keep in time

Keep in time


Nine student-musicians line up on a soccer field and clap to the beat of a metronome broadcast through a megaphone. At first view it may appear that the students farther from the metronome are not clapping in time with the metronome. However, these students are clapping when the sound of the metronome reaches their ears, delayed by the time it takes for sound to travel from the metronome.

Viewers can measure the time delay between when Emma, who is nearest the metronome, claps, and when Max, who is 80 meters away, claps. Students can use this delay to determine the speed of sound in air.


Instructor Note

When teaching with this video, direct students to the student video library, which provides student access to all videos without links to instructor materials and solutions.

Three file formats are available to work with this video. For analysis, frame-by-frame viewing is needed.

DMV player of two claps in slow motion

download video of three claps in slow motion (Quicktime Video 7MB Feb15 14)

YouTube of entire video (does not allow frame-by-frame viewing)

Teaching Materials

Classroom-ready activity:

Keep in Time Activity Page

Sample questions:

  • Why don't the students appear to clap together?
  • What's the speed of sound in air?
  • What did the student at 80 m hear?
  • What did the student at 0 m hear?
  • If you graph distance vs. time for the claps what does the slope represent?
  • If you graph distance vs. time for the claps what does the Y-intercept represent?


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