question_answer

25 tunning forks are arranged in series in the order of decreasing frequency. Any two successive forks produce 3 beats/sec. If the frequency of the first turning fork is the octave of the last fork, then the frequency of the 21st fork is [Kerala (Engg.) 2001]

A)            72 Hz                                       

B)            288 Hz

C)            84 Hz                                       

D)            87 Hz

Correct Answer: C

Solution :

                   According to the question frequencies of first and last tuning forks are 2n and n respectively. Hence frequency in given arrangement are as follows Þ 2n ? 24 ´ 3 = n Þ n= 72 Hz So, frequency of 21st tuning fork \[{{n}_{21}}=(2\times 72-20\times 3)=84Hz\]