- #1

Granger

- 168

- 7

## Homework Statement

To determine the speed of a train, a student of Physics determined the frequency of the whistle in the approach to its position of observation, and measured 220 Hz. He then determined the frequency of the whistle when the train moved away, and got 190 Hz. What is the speed of train? Consider that the velocity of sound in the air is 340 ms-1

## Homework Equations

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Doppler effect equation:

$$f' = f \frac{u+- v_Dt}{u+- v_St}$$

## The Attempt at a Solution

What I simply did was to isolate v_D to determine it.

$$v_D=v(1-\frac{f'}{f})= 340(1+\frac{190}{200}=46.4$$

Note: I choose the minus signal in the numerator because the destiny is moving away from the source.

The answer of textbook is however:

$$v_D=v(\frac{f-f'}{f+f'})= 24.9 m/s$$

I don't get what I'm doing wrong. Can someone help me?