KCET · Physics · Waves and Sound
A motor-cyclist moving towards a huge cliff with a speed of \(18 \mathrm{kmh}^{-1}\), blows a horn of source frequency 325 Hz . If the speed of the sound in air is \(330 \mathrm{~ms}^{-1}\), the number of beats heard by him is
- A \(5\)
- B \(4\)
- C \(10\)
- D \(7\)
Answer & Solution
Correct Answer
(A) \(5\)
Step-by-step Solution
Detailed explanation
Speed of motor-cyclist, \(v_s=18 \mathrm{~km} / \mathrm{h}\)
\(=18 \times \frac{5}{18} \mathrm{~m} / \mathrm{s}=5 \mathrm{~m} / \mathrm{s}\)
Frequency of source, \(f=325 \mathrm{~Hz}\)
Speed of sound, \(v=330 \mathrm{~m} / \mathrm{s}\)
Number of beats heard by motor-cyclist.
\(\Delta f=f\left(\frac{v_s}{v-v_s}\right)=325\left(\frac{5}{330-5}\right)=5\)
\(=18 \times \frac{5}{18} \mathrm{~m} / \mathrm{s}=5 \mathrm{~m} / \mathrm{s}\)
Frequency of source, \(f=325 \mathrm{~Hz}\)
Speed of sound, \(v=330 \mathrm{~m} / \mathrm{s}\)
Number of beats heard by motor-cyclist.
\(\Delta f=f\left(\frac{v_s}{v-v_s}\right)=325\left(\frac{5}{330-5}\right)=5\)
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