Gizmos Student Exploration| Doppler Shift Answer Key| Complete Solutions ADDED POSSIBLE QUESTIONS

Doppler Shift Answer Key

Vocabulary: Doppler shift, frequency, pitch, sonic boom, sound waves, wavelength

Prior Knowledge Questions (Do these BEFORE using the Gizmo.)

[Note: The purpose of these questions is to activate prior knowledge and get students thinking.Students

are not expected to know the answers to the Prior Knowledge Questions.]

Have you ever heard a siren on a moving ambulance, fire truck, or police car? If so, whathappens

to the sound as the vehicle passes by?

Answers will vary. [The pitch of the sound gets lower as the vehicle passes by.]

The change in the sound that you hear is called the Doppler shift.

Gizmo Warm-up

The Doppler Shift Gizmo™ illustrates why the Doppler shift

occurs. The Gizmo shows a vehicle that emits sound wavesand an

observer who will hear the sounds.

1. Click the PLAY SAMPLE button ( ). (Check that the

Gizmo’s sound and your computer’s speakers are on.)

What do you hear?

The pitch of the sound goes from high to low.

2. Click Play ( ) and observe the sound waves emitted from the moving car. Click Pause

( ) and compare the sound waves in front of and behind the car. What do you notice?

The sound waves are closer together in front of the car than behind the car.

3. Use the Ruler to measure the wavelength, or the distance between the lines, of the wavesin front of

and behind the car. (Note: The red circles represent every thousandth wave.)

Wavelength in front of car: ~500 m Wavelength behind car: ~900 m

[Note: The above measurements assumes that the user has chosen the default settings forthe Gizmo:

fsource = 500 Hz, vsource = 100 m/s, vsound = 340 m/s.]

4. Why do you think the waves in front of the car have a shorter wavelength than the wavesbehind

the car?

The car is moving towards the waves in front of it, and moving away from the waves behindit. As a

result, the distance between waves is shorter in front of the car than behind the car.

Activity A:

The Doppler shift

Get the Gizmo ready:

 Click Reset ( ).

 Check that fsource is set to 500 Hz and vsound is setto 340

m/s, close to the actual speed of sound.

 Set vsource to 0 m/s.

Introduction: The pitch of a sound, or how shrill or deep it is, is related to the frequency of thesound

waves. The greater the number of sound waves passing by a point each second is, the higher the

frequency and the pitch will be. The unit of frequency is the hertz (Hz).

Question: What causes the Doppler shift?

1. Observe: With the car’s velocity (vsource) set to 0 m/s, click Play. Notice the sound wavesmoving

away from the car in all directions.

A. Increase the frequency of the sound waves by moving the fsource slider to the right.

How does this affect the spacing of the waves? The waves are closer together.

When the wavelength of the waves is short, the sound will be high in pitch.

B. Now decrease the frequency by moving the fsource slider all the way to the left.

How does this affect the spacing of the waves? The waves are farther apart.

Sound waves that are spaced far apart will produce a lower, deeper pitch.

2. Measure: Click Reset. Set the frequency (fsource) to 1000 Hz. Change the velocity of the sound

source (vsource) to 200 m/s. (The car is now an airplane.) At upper right, turn on theObserved

frequency (Hz) checkbox.

Drag the observer onto the road. Click Play, and then click Pause when the sound wavesfirst reach

the observer.

A. What is the frequency of sound waves in front of the plane? ~2428 Hz [Observed

values may vary slightly depending on the position of the observer.]

B. Click Play, and then click Pause just after the plane has passed the observer. Whatis the

frequency of sound waves behind the plane? ~629 Hz

3. Summarize: Based on what you have learned, how will the sound that the observer hearschange as

the airplane passes by? Explain your answer.

As the airplane passes by, the frequency changes from 2428 Hz to 629 Hz. The change infrequency

causes the pitch of the sound to change from very high to very low.

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