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Doppler Shift Activity

Don't Loose Your Marbles-Are You Coming or Going?

In this activity the students will observe marbles rolling down a sloped track from the source (top of the track) and calculate the "frequency" of the marbles by counting how many they collect at the end of the track during a given interval. The marbles simulate wave crests. They will determine how the "frequency" changes when they collect marbles while slowly walking toward the source and then away from the source. The activity simulates the effect of a sound receiver moving with respect to the sound.

Materials

For each student:

• Copy of Student Activity: "Don't Loose Your Marbles-Are You Coming or Going?"
• Copy of Student Text: "Are You Coming or Going?" along with Sine Wave Contact Cards

For the activity:

1. Copy of Reporting/Data Sheet: "Don't Loose Your Marbles-Are You Coming or Going?"
2. A stopwatch.
3. Approximately 20 marbles or other spherical objects, such as ball bearings, that are consistent with the track that you are using. Golf balls work will with a gutter and are more easily seen by visually impaired students. Golf balls in a metal gutter also provide excellent focal points for the totally blind student to follow due to the sound being produced. The Student Activity, "Don't Loose Your Marbles-Are You Coming or Going?" is written as if marbles will be used. If this is not the case, advise the students of the change that you have made.

   Helpful Hint: In any given bag of marbles, marbles of the same size may not have the same mass. Make sure all 20 marbles weigh the same or you will introduce a variable that will change the "frequency" of the descending marbles.

4. A 15 ft. long (minimum) track along which marbles can be rolled. This might be a section of 3/4 x 3/4 inch wooden corner protector (or corner molding), available in 16 ft. lengths at stores that sell lumber. Depending upon your storage capabilities, it might be better to obtain two 8 ft. sections. These sections can be connected by hot gluing 1-1/2 inches of a 3 in. piece of curved tin to the end of one 8ft. section. An end of the second 8 ft. then can be laid on the 1-1/2 inches extending from the first piece. It helps to attach a board or other weight close to the end of the second piece to help the joints remain secure while the marbles are rolling down the track. Other tracks will also work, including gutter material, chalkboard trays, etc. You simply need to use something open on the top that will contain spherical objects as they roll down the track. The longer the track, the better.
5. An area where the track can be placed at waist to chest height of the students and supported on both ends and elsewhere, as necessary, to keep it straight. A ring stand support attached to the upper end would be very helpful in making adjustments in height. However, if ring stands are not available, wooden blocks also work fine.

Procedure

1. Before class make copies of the following:
• Student Activity, "Are You Coming or Going?"
• Student Text, "Doppler Effect"
• Reporting/Data Sheet, for "Don't Loose Your Marbles-Are You Coming or Going?"
2. You can divide the class up into teams of three and designate one team member as a "source," another as a "timer/recorder," and the third as a "receiver." Alternatively, you may wish to have the class select a "source," a "timer/recorder" and an initial "receiver" and then have each student collect his/her own data, each acting in turn as a moving receiver. In the latter case, you might also wish to rotate the "source" and "timing" responsibilities among the class so that everyone gets to collect a set of data. The exact procedure will depend on the class or how this activity fits into your schedule.
3. The "source" end of the track should be about 1 ft. higher than the "receiver" end. Students should have easy and clear access to one side of the track. Tie or otherwise clamp the track in place at the "source" end to hold the open side up. If you use wooden blocks, the track can easily be secured with hot glue.
4. It should take about 40-50 seconds to roll 20 marbles down the track when they are placed on it at roughly one-second intervals. Adjust the height of the "source" end as necessary to achieve this time frame.
5. Students should walk along the track as moving "receivers" by taking baby steps so that they cover roughly two-thirds of the length of the track in the 10 second receiving time specified. This may require them to practice a few times before collecting data. Be aware that some hand-eye coordination is required to both walk along the track slowly and pick marbles up off the track as they pass by. It may be easier for some students to use golf balls instead of marbles. With practice, even totally blind individuals can successfully complete this activity with marbles. However, golf balls do afford a better success rate. If students miss a marble and it does not get counted, they probably should start over again. The number of marbles collected while moving up and down the track does not change dramatically during the 10 second collecting period. Hence, any missed marbles can introduce significant error into the experiment.
6. Distribute copies of the Student Activity, "Don't Loose Your Marbles-Are You Coming or Going?" and a copy of the Reporting/Data Sheet to each student.
7. Instruct the students to carry out student activity. Collect their Reporting/Data Sheets.
8. Start the next period by showing tabulated student results with the appropriate medium for your students needs. Follow up with a discussion of their observations, posing questions like the following:
• In what way does the "frequency" of the marbles simulate the frequency of a sound wave?
• How does the "frequency" of the marbles compare to the frequency of real sound waves
• Can the "frequency" of the marbles be altered? Can the frequency of real sound waves be altered?
• How does the marble "frequency" change (higher or lower) as the receiver moves to and from the source?
• Why did not all students observe the same "frequency" changes?
• How would the results change if the source was moving and the receiver was stationary?
9. Instruct the students to read the Student Text, "Doppler Effect" and return their Reporting/Data Sheets to them.