Physics Ch 3 1.57

Question

A physics professor demonstrates the Doppler effect by tying a 650 Hz sound generator to a 1.0-m-long rope and whirling it around her head in a horizontal circle at 120 rpm. Assume the room temperature is 20degrees C.

What is the highest frequency heard by a student in the classroom?(in HZ)

What is the lowest frequency heard by a student in the classroom?

Answer 1

The speed of sound at 20°C is about 343 m/s.

The orbital speed of the sound generator is

(1.0 m)(120 rev/min)(2π rad/rev)(1 min/60 s) ≈

12.56637 m/s

λo ≈ (343 m/s) / 650 Hz) ≈ 0.5276923 m

What is the highest frequency heard by a student in the classroom?

(343 + 12.56637) / 0.5276923 ≈ 674 Hz

What is the lowest frequency heard by a student in the classroom?

(343 - 12.56637) / 0.5276923 ≈ 626 Hz