What is the perceived frequency of Bird B when it hears Bird A's screech?

[anotherperson]

Homework Statement

Bird A and B are flying towards each other
Bird B at 20m/s
Bird A at 15m/s and screeches with a frequency of 3200 Hz
what does bird B perceive?

Homework Equations

the speed of sound is 343m/s

Fobserver==Fsource((vsound+vobserver)/(vsound+vsource)

The Attempt at a Solution

fobserver=3200((343+20)/(343+15))
=3244.7Hz

i check this against my solution and it appears to be wrong but I am not sure why. is it i got observer and source mixed around or maybe my signs?

 

[ideasrule]

Yeah, you mixed up the signs. vobserver should be positive if the receiver is moving towards the source. vsource should be positive if the receiver is moving away from the source.

You can remember this by remembering that frequency always goes up if the receiver is moving towards the source. Hence, the numerator must get better, and the denominator must get smaller.

 

[anotherperson]

so in reference to this question is should be 20 and -15?

 

[ideasrule]

Yup.

 

[anotherperson]

thanks!