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Question:

5 beats/ second are heard when a turning fork is sounded with a sonometer wire under tension, when the length of the sonometer wire is either 0.95m or 1m. The frequency of the fork will be:

300Hz

251Hz

195Hz

150Hz

Solution:

L1=0.95m,L2=1m
L2>L1,n1>N>n2
n1−N=5 and N−n2=5
on solving
n1−n2=10
n2=n1−10
By law of length of vibrating string
n1L1=n2L2
On solving we get
n1=200Hz
n1−N=5
N = 195 Hz