A boy on a bicycle approaches a brick wall as he sounds his horn at a frequency 400.00 Hz. The sound he hears reflected back from the wall is at a frequency 408.00 Hz. At what is the speed is the boy riding his bicycle toward the wall? Assume the speed of sound in air is 340 m/s.
A) 3.68 m/s
B) 333 m/s
C) 6.67 m/s
D) 6.80 m/s
E) 3.37 m/s

Question

Quizplus · Accepted Answer

The Doppler effect formula for a source moving towards a stationary observer is used here. The frequency heard by the boy is higher due to his motion towards the wall. Using the formula f' = f(v + v0) / v, where f' is the observed frequency (408 Hz), f is the source frequency (400 Hz), v is the speed of sound (340 m/s), and v0 is the speed of the source (the boy), we solve for v0 to find it is approximately 3.37 m/s.

A Boy on a Bicycle Approaches a Brick Wall as He