Physics 9B-C Quiz #3 Name: 10/17/2008 ID-last 4: A woman stands at rest in front of a large smooth wall. She holds a vibrating tuning fork of frequency fø directly in front of her (between her and the wall. The woman now runs toward the wall with speed v w . Sound waves travel with speed v a . She detects beats due to interference between sound waves reaching her directly from the fork and those reaching her after being reflected from the wall. a) (7 pts) How many beats per second will she hear? Use f L = v a ± v L v a v s f s toward away ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ This involves a double Doppler shift. The sound that arrives at wall and is reflected (R) is doppler shifted up in frequency because the source, the woman (L) is moving toward wall (W). Let the frequency of the tuning fork be
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