Warm-Up Exercise for lecture 26

Due 8:00 am, Thurs, Dec 4

Physics 105, Fall 2008

If a loudspeaker emits spherical sound waves in all directions, what decreases as you go farther away from the loudspeaker?
☐ frequency
☑ intensity
☐ wavelength

You go to a rock concert where the sound level where you are standing is 110 dB. How does the intensity (power/area) of sound waves compare to when you listen to the same music on your home stereo system, 90 dB at the spot you sit?
☐ Concert intensity = Stereo intensity
☐ Concert intensity = 1.22 stereo intensity
☐ Concert intensity = 2 stereo intensity
☐ Concert intensity = 10 stereo intensity
Concert intensity = 20 stereo intensity
Concert intensity = 100 stereo intensity

You hear a sonic boom:
☐ when an aircraft above you first exceeds the speed of sound ("breaks the sound barrier")
☑ whenever the aircraft flies overhead faster than the speed of sound.

Ralph recently saw this bumper-sticker on a professor's car: "If this sticker is blue, you're driving too fast!" (True story: a professor at my previous university had the sticker on her car.) The sticker looks RED to him. He is confused. Can you explain the joke to Ralph? (Note: Light undergoes a Doppler effect similar to sound, although the precise equation is slightly different than the one for sound given in the textbook. Also, you need to know that red light and blue light both travel at c = 3108 m/s, but red light has a longer wavelength than blue.)

Due to the Doppler effect, frequency shifts up when an observer (the car in back of you) moves towards a source (your bumper sticker). The joke here, is that with light you would only get a shift significant enough to change red to blue when the observer is moving at a substantial fraction of the speed of light.

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