Question

Two cars, one in front of the other, are travelling down the highway at 25 m/s. The car behind sounds its horn, which has a frequency of 640 Hz. What is the frequency heard by the driver of the lead car? (Vsound=340 m/s).
The answer choices are:

A) 463 Hz
B) 640 Hz
C)579 Hz
D) 425 Hz
E) 500 Hz

Answer 1

Answer:

$f_s = 640 Hz$

Explanation:

For this case we know that the speed of the sound is given by:

$V_s = 340 m/s$

And we have the following info provided:

$v_c = 25 m/s$ represent the car leading

$v_s= 25 m/s$ represent the car behind with the source

$f_o = 640 Hz$ is the frequency for the observer

And we can find the frequency of the source $f_s$ with the following formula:

$f_s = \frac{v-v_o}{v-v_s} f_o$

And replacing we got:

$f_s = \frac{340-25}{340-25} *640 Hz = 640 Hz$

So then the frequency for the source would be the same since the both objects are travelling at the same speed.

$f_s = 640 Hz$