# Doppler Effect of submarines

1. Jan 29, 2006

### BoogieL80

I'm working on the following problem:

Two submarines are underwater and approaching each other head-on. Sub A has a speed of 4 m/s and sub B has a speed of 18 m/s. Sub A sends out a 1010 Hz sonar wave that travels at a speed of 1522 m/s.

The question asks two questions, but I'm stuck on the first one:

What is the frequency of the sound detected by sub B (to the nearest Hz)?

I was using the formula frequencyobserver = frequencysource * 1 + (vobserver / velocity of sound) / 1 - (vsource / velocity of sound). I assumed that the numerator would use the positive sign since the observer is moving towards the source. I used the minus sign in the denominator since the source is moving towards the observer. For the speed of sound I used 1522 m/s since the submarines are travelling in salt water. I then multiply my calculated numbers times 1010 and got a frequncy of 1019 Hz. However webassign is telling me this is incorrect. Any help with what I'm doing wrong?

2. Jan 29, 2006

### Psi-String

Maybe you pluged in wrong data. Your equation looks correct to me.

$$f' = f \frac{v+v_D}{v-v_S}$$

where
$$v= 1522 m/s$$

$$v_D = 18 m/s$$

$$v_S = 4m/s$$

And my answer is f'=1024.6 Hz

3. Jan 29, 2006

### BoogieL80

My goodness, that thing was too picky. I put in 1023 Hz the first time and it didn't accept the number. Thank you for your help.