# Sound Waves Question

1. Dec 7, 2004

A stationary motion detector sends sound waves of 0.700 MHz toward a truck approaching at a speed of 38.0 m/s. The speed of sound in the air is 343 m/s. What is the frequency of the waves reflected back to the detector?

I tried doing F=f((v+vo)/(v)), which becomes F=.7MHz((343+38)/(343)), and this gives the wrong answer. Then I realized that something should be doubled becomes the waves are reflected twice, from the detector to truck and then from the truck to the detector, so I just double the answer I got previously and this is wrong. What am i doing wrong?

2. Dec 7, 2004

### Andrew Mason

Since the objects are approaching, you have to use: $$f_{apparent} = f_{source} \frac{v_{sound}}{v_{sound} - v_{source}}$$

You then have to apply it again using the $f_{apparent}$ as the source frequency. The difference is not exactly doubled in doing this.

AM