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### Course: Class 11 Physics (India)>Unit 19

Lesson 7: The Doppler effect

# Doppler effect formula when source is moving away

Continue your exploration of the Doppler effect by looking at the case where the source is moving away from the listener. Understand the formulas for observed frequency when the source is moving towards or away from the observer. Apply these concepts to real-world scenarios like the changing pitch of a moving train's horn. Created by Sal Khan.

## Want to join the conversation?

• so, what will the formula be if the observer is in an angle from the source's direction of motion ( not exactly in front of or behind the source)?
• I would think that it would be cos of angle that the velocity of the source makes with the observer.
• For both of the equations, why must we divide by Vw ?
To = Ts ((Vw - Vs) / Vw)
• In this case, Ts(Vw-Vs) is the distance between the source and the observer after the source moved. Mathematically speaking, Time=Distance/Speed; which corresponds with this situation because To is the time, Ts(Vw-Vs), and Vw is the speed of the sound wave.
• What happens if the observer is traveling at the exact same speed as the wave from the source or the source itself? What does the observer experience?
• yeah does the observer seem as if he remains the same as in position or does his angle change with the motion
• Can the observer sitting in the source emitting sound, if traveling faster than the speed of sound, hear the source sound normally?
• No, if the observer is moving faster than the speed of sound, then the wave being emitted will never reach him, assuming his speed is constant. lets say the wave moves at a speed of 8m/s for example but if the source moves at 10m/s the source would be 2 meters in front of the wave within the time interval of 1 second.
• Can this change in frequency also be observed in electromagnetic waves?
• Yes, it's called red-shift or blue-shift, depending on if the frequency is lower or higher.
• Why speed of the source doesn't affect speed of the wave ?
And this is a law or an assumption ?
• The speed of a wave depends on the medium it is travelling in. This is a law.
It is a good question though, because the speed of the source is important for other things.
For instance, since the speed of the wave depends on the medium, it is constant if the medium does not change, but the source can speed up. Some aircraft are capable of going faster than the speed of sound. When they do this the actually catch up to the sound waves they emitted earlier, and emit more sound waves at the same time, eventually they can break through its own sound waves, and when this happens a very loud sonic boom can be heard. If the airplane is not strong enough then this sonic boom can even break the plane.
• Why does the frequency of an approaching object seem to be increasing gradually, and then decrease gradually? From the diagram above it seems that frequency suddenly drops as the source passes you by.
• It does suddenly drop. Listen to what it sounds like as a car goes by you. The change is very gradual until the car is close. As it goes by, the change is very fast.
• Will the same formula hold true if the observer is not exactly in the path of source but sideways?
• If you mean the source is moving straight and the observer is stationed at an angle from it, this was an answer given by Andrew M for a similar question:
The same concept holds true but now you just have to use trigonometry to get the velocity component that is parallel to the movement of the source given the angle between the source's path and the listener's location.
• at 8.00 I don't understand why this expression (10ms / 10ms-5ms) isn't evaluated in a way that the 10 m/s over 10 m/s doesn't jcancel out leaving you with 1 / 1-5.

Instead he seems to do the denominator first 10ms-5ms giving him 10ms / 5 ms = 2 cycles. Is their a rule I've missed out when evaluating in fractions? because I always assumed that multiplication and division took priority over addition and subtraction.
(1 vote)
• If you look at what is written the division line is over both the 10 - 5 so it is 10/(10 -5) not 10/10-5.