The Doppler Effect

Doppler Effect Objectives

Measure the detector frequency for waves emitted from a slowly moving source as that source is approaching the detector. (Exploration 1)

Calculate the detector frequency for waves emitted from a slowly moving source as that source is moving away from the detector. (Exploration 2)

Sketch the wave-front patterns for wave sources with various source speeds. (Exploration 3)

Description of Activity In this activity, you will study waves that travel from a moving source to a detector. You will control the source speed as well as the frequency of waves emitted by that source.

You will observe the wave fronts and measure the frequency at the detector. The Jump Start exercises below will help you review frequency, wavelength, pitch, and the Doppler effect. Jump Start

1. What type of wave is a sound wave? A sound wave is a longitudinal waves.
2. Define wave frequency.

• Wave frequency is the number of crests that pass through at a specified time.
• What is pitch? A pitch is the sound or sensation of the frequency.
• Sketch one wavelength of a longitudinal wave.

Exploration

A Wave Source Moving Towards a Detector Procedure

Explore the simulation on your own for several minutes. Attempt to identify relationships among source frequency, detector frequency, wave speed, and source velocity. Set Source speed to 1. 0 cm/s. Move the detector by dragging it from the left side of the screen onto the grid; place it on the right side of the grid, directly opposite the wave source. Set Wave speed to 5. 0 cm/s. Select a Source frequency. Record this frequency in Table 1. The top stopwatch in this Virtual Investigation starts automatically when the first wave front touches the detector.

The second stopwatch does not start until the source has passed the detector. Select Go. Using the top stopwatch, observe the number of waves that pass the detector in 1. 0 s. This is the detector frequency. Record this frequency in Table 1. In addition, sketch the wave-front pattern on a separate sheet of paper. 4. Repeat step 3 for at least two more trials. Keep Source speed, Wave speed, Source frequency, and detector position the same for all three trials. Repeat steps 2 through 4 for at least three more source frequencies Observations and Analysis

For each source frequency, average the detector frequencies. Record these averages in Table 1. 2. Are the source frequencies greater than, less than, or the same as the detector frequencies in this Exploration? The source frequencies were less than the detections.

Exploration 2: A Source Moving Away from a Detector Procedure

Set Source speed to 1. 0 cm/s and Wave speed to 5. 0 cm/s. Place the detector on top of the source. Set Source frequency to any value. Record this source frequency in Table 2. This time, the detector will detect waves as the source moves away from it. Select Go. In Table 2, record the number of wave fronts that pass the detector in 5. 0 s. 4. Repeat steps 2 and 3 for at least three more source frequencies. Observations and Analysis Table 2 (source speed = 1. 0 cm/s; wave speed = 5. 0 cm/s)

Divide the number of times that the detector light flashes in 5. 0 s by 5. 0 for each source frequency in Table 2. This is the detector frequency. In Table 2, record the detector frequency for each source frequency. Are the source frequencies greater than, less than, or the same as the detector frequencies in this Exploration? The detector frequencies are greater than the source frequencies. In Exploration 1, you averaged the results of three trials.

In Exploration 2, you gathered data over a longer period of time. Which approach probably yielded more accurate results? Why? I think Exploration 1 yielded more accurate results because the detector was not sitting above and it gave the detector an accurate reading.

Exploration 3: A Moving Source at Different Velocities Procedure

Set Wave speed to 10. 0 cm/s and Source frequency to 1. 0 Hz. Place the detector anywhere. Set Source speed to 6. 0 cm/s. 3. Select Go. Sketch the resulting wave-front pattern on a separate sheet of paper. Set Source speed to 8. 0 cm/s. 5.

Select Go. Sketch the resulting wave-front pattern on the separate sheet of paper. 6. Repeat steps 4 and 5 for 10. 0 cm/s, 12. 0 cm/s, and 14. 0 cm/s source speeds.

Observations and Analysis

What happens to the wave-front pattern as the source speed is increased to equal the wave speed? The amount of waves seen in a given time seems to increase and reach the detector much faster. What happens to the wave-front pattern as the source speed is increased beyond the wave speed? When the source speed is increased beyond the wave speed the waves frequency is extremely high. Conclusions

Describe how the motion and frequency of a wave source affects the waves that source produces. When the frequency and motion are both set at high rates, the waves that are produced and their frequency is increased. When the motion and frequency are decreased the waves decrease as well. Inquiry Extension Luisa is swinging on a playground swing at school. A teacher facing her blows a whistle to let the children know recess is over. As Luisa swings, what does she hear? When does she hear the highest pitch? As Luisa swings she hears the whistle, but she hears the highest pitch when she is swinging away from the teacher.