Crocodile Physics Simulation: a moving fire engine

Watch the YouTube video, pay close attention to the sound of the fire engine as it passes the camera.

Did you notice the change in the pitch of the siren as the fire engine drives by? You will be able to understand why it is so by the end of this lesson.

Download and open this crocodile physics file, and complete the following worksheet (clone and upload).

What affects the pitch of a sound?

Now, view Scene 1 of the demostration, the point on the left end of the ruler is a source of sound. It is much like the fire engine, except it's stationary, for now.

unpause the simulation, observe the sound wave travels in the space. After a few seconds, pause the simulation. you should have a screen that looks like this

The sound wave travels in all directions, but for this exercise, we are only interested in the part of wave travelling in the horizontal direction. what is the wavelength of this sound?

Given  the speed of sound in air is 340m/s. what is the frequecy of this soundwave?

Now switch to scene 2. this is where we make the "fire engine" move. The point source is now given a horizontal speed. (in this case, we let it move at 40% of the speed of sound, which is way faster than a fire engine. This is so that it is easier to compare the difference) unpause and run the simulation for a few seconds; then pause it again so that you have a few cycles of waves to the left and right of the source. It should look something like this.

Observe the pattern carefully. Are there any difference in the pattern of the wave on either side of the source?

Measure the respective wavelength of the sound on both side of the source, and calculate the frequencies.

How will the pitch of the sound be to the left and right of the moving source?

Scene 3 is an automated version. Two detectors are placed on the left and right end of the measuring rule. the frequencies of the sound is detected and plotted on the graph. 

the point is initially at rest. unpause the simulation and wait until both detector recorded a wave, like this:

Pause the simulation and study the graph. what are the frequencies on the both detectors? how do they compare to your calculation in scene 1?

Change the source speed to 137.44m/s (the same value as scene 2). Unpause the simulation until change in frequency are observed on both detectors, then pause it again. The simulation should look something like this:

Study the new graph: what are the frequencies at both detectors? How does the simulation result compare to your answer in scene 2?

What does the frequency tells you about the pitch of the sound?

Watch the video again. Is your simulation result consistent with the phenomenon observed (heard) on the video? If not, why not?

To solve the final mystery, watch Scene 4, then make a brief explaination.

So here, change of frequency give the sound a changing pitch.

FYI, this effect is called Doppler Effect, and you will get to know more about it in "A" level physics.

NEXT: sample answer