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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