What Frequency Do You Hear from a Speeding Train's Whistle?

[Dx]

A train is traveling toward you at 120km/h. The train blows its 400Hz whistle. take the speed of sound to be 340m/s. what's the frequency do you hear?

f'=f/(1-vs/vo)
400/(1-(4.32x10^8 / 340)) i got 4.32x10^8 = 120000m * 3600sec

what am i doing wrong?
The possible answers are: which is it and what have i done wrong to miss this test question?

a) 444hz
b) 364hz
c) 361 hz

thanks!
Dx

 

[quantumdude]

Originally posted by Dx
f'=f/(1-vs/vo)
400/(1-(4.32x10^8 / 340)) i got 4.32x10^8 = 120000m * 3600sec

You made a mistake converting the speed of the source from km/h to m/s. If you look at it, you have the source moving faster than light, which is a big no-no.

 

[M98Ranger]

Based on the given information, the correct answer is b) 364hz. To arrive at this answer, you have to use the formula for frequency of a moving source, which is f'=f/(1-vs/vo). In this case, f is the original frequency of the whistle, which is 400Hz. vs is the speed of the source, which is the train moving at 120km/h or 33.33 m/s. vo is the speed of sound, which is given as 340m/s. So, the equation becomes f' = 400/(1-33.33/340) = 364hz. Therefore, the frequency you hear is 364hz. You may have made a mistake in your calculations, which is why you got a different answer. Make sure to double check your numbers and use the correct formula to arrive at the correct answer.