A car is moving at a constant velocity. While you are stationary, you hear the car approaching and determine the sound you hear is at 1369.75 Hz.

Question

After the car passes, you measure the sound from the car at 1149.5 Hz.Assume the car was producing a constant sound for the entire time. How fast was the car traveling? What frequency sound was the car producing? Please show your work.

(please explain) This is Physics 1

Answer 1

The approach to this problem is the set up the two instances - frequency of the approaching sound and the frequency of the receding sound. The equations are:

F(approaching) = c/(c-v) Factual and F (receding) = c(c+v) F(actual)

c is the speed of sound - I use 343 m/s

v is the velocity of the sound.

F is the frequency (approaching, receding, or actual)

Dividing the left equation by the right and plugging in the numbers,

1369.75/1149.5 = (343 - v)/343+v), I get about 30 meters/sec.

You can plug the value of v into either of the above equations to get the actual frequency at about 1250 Hz