Calculate Horn Frequency Decay Time in Seconds

[05holtel]

Homework Statement

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.

Give your answer in s, and to three significant figures.

Only answer in numerical values, no units.

Homework Equations

speed of sound in air is 340 m/s

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

 

[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.

 

[05holtel]

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

Thanks alot

 

[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.