Q. 25 tuning forks arranged in series in the order of decreasing frequency. Any two successive forks produce 3 beats/sec. If the frequency of the first tuning fork is the octave of the last fork, then the frequency of the 21$^{st}$ fork is
A) $72\ Hz$
B) $288\ Hz$
C) $84\ Hz$
D) $87\ Hz$
Explanation
(c) According to the question frequencies of first and last tuning forks are $2n$ and $n$ respectively.
Hence frequency is given arrangement are as follows
[image:15_image67.png]
$\Rightarrow 2n - 24 \times 3 = n \Rightarrow n = 72\ Hz$
So, frequency of 21$^{st}$ tuning fork
$n_{21} = (2 \times 72 - 20 \times 3) = 84\ Hz$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.