# Frequency of Horn

1. Jan 28, 2010

### 05holtel

1. The problem statement, all variables and given/known data

While blowing its horn of frequency 451 Hz, a car accelerates at 0.64 m/s2.

The car starts from rest by your side and moves away.

How long will it take for the frequency you hear to decrease by 5%?

Hint: Do not forget to include the time it will take for sound to reach you.

Only answer in numerical values, no units.

2. Relevant equations

speed of sound in air is 340 m/s

3. The attempt at a solution

fsource = fo (1/(1+vsource/c))

0.95 = (1/(1+vsource/c))

vsourve = 17.89m/s

V= v0 +at
t= 17.89/0.64
t = 28.0 seconds

This is wrong

2. Jan 28, 2010

### Gear300

You probably just need to take into account the time it takes for sound to reach the observer. If the time you got is right, then at 28.0s the car would have traveled some distance that sound would have to cover in some time to get to the observer.

3. Jan 28, 2010

### 05holtel

OK. I am not sure how to account for this. What equation would i use?

Thanks alot

4. Jan 28, 2010

### Gear300

The car traveled a distance x = .5at2 from the observer, in which t = 28.0s. This would mean that for the sound carrying the relative frequency to get to the observer, it would have to travel through this distance to get to the observer. Your total time would take this into account.