# Warm-Up Exercise for lecture 27

## Due 8:00 am, Tues, 8 Dec 09

Physics 105, Fall 2009

If two waves are shifted by ________, completely destructive interference will occur.
l/2
2l/3
l
2l

In a standing wave, the points that have the maximum vibration are called:
☐ nodes
☑ anti-nodes

You have two pipes which produce sound: one is open at both ends (like an organ pipe) and the other is open at only one end (like a panpipe). If the two pipes have the same length, the fundamental resonant frequency will be ____________ for the two.
☐ the same
☑ different

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.