![]() ![]() This is known as relative motion, the speed of objects depends on the frame from which they are measured. If on the contrary you were moving in the same direction as the train, the train would appear to move slower, or even stationary if your speed is identical to that of the train. The train also appears to be moving faster than it does when you are stationary. Of course, the train itself is moving at a constant speed, but according to the observer the train is moving faster. But the frequency with which you are seeing each new car as you are moving in the opposite direction of the train is greater than if you were stationary. The distance between the cars of the train appears the same to you, so the wavelength is the same. ![]() The observer is you in the moving vehicle, and the train represents the moving wave. Each observer hears the same frequency, and that frequency is. For example, if you ride a train past a stationary warning horn, you will hear the horn’s frequency shift from high to. Although less familiar, this effect is easily noticed for a stationary source and moving observer. Consider two stationary observers l ocated on either side of a stationary source. The Doppler effect is an alteration in the observed frequency of a sound due to motion of either the source or the observer. Doppler shifts occur in the frequency of sound, light, and water waves. Let us think of this in terms of an analogy. Imagine you are driving in a vehicle with a train on tracks parallel to the road approaching you. The Doppler effect occurs not only for sound, but for any wave when there is relative motion between the observer and the source. But how is this possible if the wavelength remains the same and the medium does not change? Thus, the frequency according to the observe increases. If the blue dot representing the observer moves toward the source in Figure 8.4.1, it would flash more frequently since it would encounter crests more often, than it would if stationary. A moving observer would measure the same distance between crests as it would if it was stationary. Thus, in this scenario the wavelength is fixed. In this case since the source is stationary the distance between the crests is the same in front and behind the source, as shown in the animation of Figure 8.4.1. The Doppler effect is an alteration in the observed frequency of a sound due to motion of either the source or the observer. Let us consider what happens when, instead of the source, the observer is moving toward or away from a stationary source. Where the sign is a plus for a source moving away from the observer and a minus for a source moving toward the observer.
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