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# Doppler Effect

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Doppler Effect

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### Doppler Effect

1. 1. The Doppler Effect The Concept Explained!
2. 2. ● The phenomena that the wave frequency changes when the distance between a source of sound and a receiver of sound: o If the objects are coming closer to one another, the frequency increases o If they are moving farther away from each other, the frequency decreases. Introduction http://www.redorbit.com/media/upl oads/2004/10/6_2794e0fcae75d8 49e10a80c3137aa8bc2.jpg
3. 3. There are three cases related to the doppler effect that can be modeled by a simple equation. ● stationary source + moving receiver ● stationary receiver + moving source ● moving source + moving receiver Cases of Doppler Effect
4. 4. The equation used to model these 3 cases: Equation Vr = velocity of the receiver Vs = the velocity of the source fr = the frequency of the receiver fs = the frequency of the source Note the ± and ∓ in the numerator and denominator of the equation. ● Use the top sign of the numerator if the receiver is moving towards the source. There is an observed increase in frequency ● Use the top sign of the denominator if the source is moving towards the receiver. There is also an increase in frequency observed.
5. 5. For the next couple of slides we will be looking at how the doppler effect occurs in the sounds of an emergency vehicle siren. Emergency Vehicle Siren and You
6. 6. Let’s Pretend that the Source was Stationary ● emits spherical waves with the same speed in all directions ● Receiver at any point will detect the same frequency because they are equally spaced apart http://cfcpwork.uchicago.edu/kic p- projects/nsta/2007/sherman/dop pler_files/image004.png
7. 7. 1) Moving Source + Stationary Receiver o Imagine that an emergency vehicle is approaching you from the left and continues to move right. Special Cases when vr= 0 or vs = 0 http://mail.colonial.net/~hkaiter/aa_newest_images/doppler.effect.diagram.jpg
8. 8. ● Distances between wave fronts at the right side of the source are closer together than the wave fronts at the left of the source ● Waves are still travelling at the same speed. ● Smaller distance between the wave fronts on the right, a receiver would detect more waves per second = higher frequency http://cfcpwork.uchicago.edu/kicp- projects/nsta/2007/sherman/doppler_fi les/image004.png
9. 9. 2) Moving Receiver + Stationary Source ● receiver detects a reduction in the wave speed Special Cases when vr= 0 or vs = 0
10. 10. First let’s think of it in this way: Let’s pretend that the receiver was moving towards the stationary source! Let’s look at this from a different approach:
11. 11. ● If the receiver was one crest away from away from the source, the distance would be one wavelength away from the source ● the speed from the perspective of the receiver would be v+vr since it would experience more waves per second ● The numerator would have a minus sign when we plus this into the equation
12. 12. In the end we get: Stationary Source Stationary Receiver Upper sign = motion towards Lower sign = motion away