# Homework Help: Doppler Effect help!

1. Mar 13, 2007

### ku1005

Hey guys...or ladies....um....just wondering what I hav done wrong for the following Q, it is an online Q i am doing and it tells me my answer is incorrect???

Q) "A car alarm is emitting sound waves of frequency 520 Hz. You are on a motorcycle, traveling directly away from the car."

"How fast must you be traveling if you detect a frequency of 490 Hz?
Enter your speed as a positive number. Take the speed of sound to be 344 m/s.

It seems fairly striaght forward, so I used the doppler effect (ie the wavelength must be greater and hence the frequency less if moving away)

so...

f(effetive) = v/(v+vl) . f

where vl = velocity of listener...

so i have

344/(344 . vl) . 520 = 490

solving for vl gives....21.06 ms^-1 roughly

so therfore, bke must be moving at 21.06ms^-1 ?

BUT it tells me this answer is incorrect??

any ideas appreciated!!

cheers

rhys

2. Mar 13, 2007

### haiha

Nothing is wrong here. You may have to check if the car is stopdead or it may be moving at a certain speed.

3. Mar 13, 2007

### ku1005

mmm....program must b dodgi....cheers