***Doppler effect . What is the Doppler effect?

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Transcript of ***Doppler effect . What is the Doppler effect?

  • Slide 1
  • ***Doppler effect
  • Slide 2
  • What is the Doppler effect?
  • Slide 3
  • NEXRAD
  • Slide 4
  • Stationary observers hear the frequency of the source.
  • Slide 5
  • Can you estimate how fast this thing is moving through the air?
  • Slide 6
  • Source moving at subsonic speeds (Mach 0.7 )
  • Slide 7
  • Moving firetruck (source)
  • Slide 8
  • Moving source, stationary observer ' = - v s T
  • Slide 9
  • Moving observer, stationary source
  • Slide 10
  • Doppler effect can involve a moving source, a moving receiver, or both.
  • Slide 11
  • ***Doppler effect can involve a moving source, a moving receiver, or both.
  • Slide 12
  • Why do these curves behave differently?
  • Slide 13
  • A high-speed train is traveling at a speed of 44.7 m/s (100 mi/h) when the engineer sounds the 415-Hz warning horn. The speed of sound is 343 m/s. What are the frequency and wavelength of the sound, as perceived by a person standing at a crossing, when the train is (a) approaching and (b) leaving the crossing?
  • Slide 14
  • A train sounds its whistle as it approaches a tunnel in a cliff. The whistle produces a tone of 650 Hz. If it travels at a speed of 21.2 m/s, what is the frequency heard in the tunnel and in the reflected sound?
  • Slide 15
  • A bat flies toward a moth at speed 6.7 m/s while the moth is flying toward the bat at speed 5.5 m/s. The bat emits a sound wave of 51.99 kHz. What is the frequency of the wave detected by the bat after it reflects off the moth? (Assume the speed of sound is 343 m/s.)
  • Slide 16
  • Mission accomplished!
  • Slide 17
  • A siren, mounted on a tower, emits a sound whose frequency is 2140 Hz. A person is driving a car away from the tower at a speed of 27 m/s. The sound reaches the person by two paths: the sound reflected from a building in front of the car, and the sound coming directly from the siren. What frequency does the person hear for the (a) reflected and (b) direct sounds? f o = f o (1 v 0 /v) = 2140(1 27/343) = 1970 Hz f o = f o (1 + v 0 /v) = 2140(1 + 27/343) = 2310 Hz
  • Slide 18
  • Two loudspeakers are mounted on a merry-go-round whose radius is 9 m. When stationary, the speakers both play a tone whose frequency is 100 Hz. As the drawing illustrates, they are situated at opposite ends of a diameter. If the merry-go-round revolves once every 20 s, what is the beat frequency that is detected by the listener when the merry-go-round is near the position shown?
  • Slide 19
  • Suppose a police radar gun emits an electromagnetic wave with a frequency of f s = 8.010 9 Hz. The car in question is approaching a police car parked on the side of the road in an essentially head-on fashion. The wave from the radar gun reflects from the speeding car and returns to the police car, where on-board equipment measures its frequency to be greater than that of the emitted wave by 2100 Hz. Find the speed of the car with respect to the highway.
  • Slide 20
  • ***Moving medium (wind)
  • Slide 21
  • Two trains are traveling toward each other at 16 m/s relative to the ground. One train is blowing a whistle at 1320 Hz. (a) What frequency will be heard on the other train in still air? (Assume the speed of sound in air is 343 m/s.) (b) What frequency will be heard on the other train if the wind is blowing at 16 m/s toward the whistle and away from the listener? (c) What frequency will be heard if the wind direction is reversed?
  • Slide 22
  • Two train whistles, A and B, both have a frequency of 386 Hz. A is stationary and B is moving toward the right (away from A) at a speed of v B = 33.0 m/s. A listener is between the two whistles and is moving toward the right with a speed of 14.1 m/s. Assume that the speed of sound in still air is 343 m/s and that no wind is blowing. What beat frequency is detected by the listener?
  • Slide 23
  • A girl is sitting near the open window of a train that is moving at a velocity of 23 m/s to the east. The girl's uncle stands near the tracks and watches the train move away. The locomotive whistle emits sound at a frequency of 580 Hz. Assume that the speed of sound in still air is 343 m/s. (a) What frequencies do the girl and uncle hear? (b) How would your answers change if a wind now blows from the east at 9.2 m/s.
  • Slide 24
  • End