# Doppler Effect Help: Solving Car Alarm Q with 21.06 m/s Speed

• ku1005

#### 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

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

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