If a loudspeaker emits spherical sound waves in all directions, what
decreases as you go farther away from the loudspeaker?
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´
Concert intensity = 2´
Concert intensity = 10´
Concert intensity = 20´
Concert intensity = 100´
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 = 3´108 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.