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

`f_s = 640 Hz`

Step-by-step 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 = (v-v_o)/(v-v_s) f_o`

And replacing we got:

`f_s = (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`