# Doppler Effect?

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You are standing on a train station platform as a train goes by close to you. As the train approaches, you hear the whistle sound at a frequency of f1 = 92 Hz. As the train recedes, you hear the whistle sound at a frequency of f2 = 79 Hz. Take the speed of sound in air to be v = 340 m/s.

Find the numeric value, in hertz, for the frequency of the train whistle fs that you would hear if the train were not moving.

You are standing on a train station platform as a train goes by close to you. As the train approaches, you hear the whistle sound at a frequency of f1 = 92 Hz. As the train recedes, you hear the whistle sound at a frequency of f2 = 79 Hz. Take the speed of sound in air to be v = 340 m/s.

Find the numeric value, in hertz, for the frequency of the train whistle fs that you would hear if the train were not moving.

##### 1 Answer

#### Explanation:

As an estimate I would expect the frequency to be about halfway between the two values i.e

We can show this as follows:

When the train is approaching we get:

Where v is the speed of sound.

We can find the speed of the train

When the train is receding we get:

Putting in the numbers we get:

Now divide

This is the speed of the train.

Now we can substitute this value back into

From the prediction this seems reasonable.