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(source moving toward observer) (17.11) That is, the observed frequency is increased whenever the source is moving toward the When the source moves away from a stationary observer, as is the case for observer B in Figure 17.9a, the observer measures a wavelength (. that is greater than ( and hears a decreased frequency: (source moving away from observer) (17.12) We can express the general relationship for the observed frequency when a source is moving and an observer is at rest as Equation 17.11, with the same sign convention S as was applied to v O : a positive value is substituted for v S when the source moves toward the observer and a negative value is substituted when the source moves Finally, we find the following general relationship for the observed frequency: (17.13) In this expression, the signs for the values substituted for v O and v S depend on the di- rection of the velocity. A positive value is used for motion of the observer or the source A convenient rule concerning signs for you to remember when working with all Doppler-effect problems is as follows: f . # ! v $ v O v * v S "
f f . # ! v v $ v S "
f f . # ! v v * v S "
f S E C T I O N 17. 4 • The Doppler Effect 525 ▲ PITFALL PREVENTION 17.1 Doppler Effect Does Not Depend on Many people think that the The word toward is associated with an increase in observed frequency. The words Although the Doppler effect is most typically experienced with sound waves, it is a phenomenon that is common to all waves. For example, the relative motion of source Quick Quiz 17.7 Consider detectors of water waves at three locations A, B, and C in Figure 17.9b. Which of the following statements is true? (a) The wave speed is Quick Quiz 17.8 You stand on a platform at a train station and listen to a train approaching the station at a constant velocity. While the train approaches, but be- General Doppler-shift expression |