MCQ
Answer the following by appropriately matching the lists based on the information given in the paragraph. A musical instrument is made using four different metal strings, $1,2,3$ and $4$ with mass per unit length $\mu, 2 \mu, 3 \mu$ and $4 \mu$ respectively. The instrument is played by vibrating the strings by varying the free length in between the range $L _0$ and $2 L _0$. It is found that in string$-1(\mu)$ at free length $L _0$ and tension $T _0$ the fundamental mode frequency is $f _0$.
List$-I$ gives the above four strings while List$-II$ lists the magnitude of some quantity.
List$-I$ List$-II$
$(I)$ String$-1( \mu$ ) $(P) 1$
$(II)$ String$-2 (2 \mu)$ $(Q)1 / 2$
$(III)$ String$-3 (3 \mu)$ $(R)1 / \sqrt{2}$
$(IV)$ String$-4 (4 \mu)$ $(S)1 / \sqrt{3}$
  $(T)3 / 16$
  $(U)1 / 16$
$(1)$ If the tension in each string is $T _0$, the correct match for the highest fundamental frequency in $f _0$ units will be,
$(1)\ I \rightarrow P , II \rightarrow R , III \rightarrow S , IV \rightarrow Q$
$(2)\ I \rightarrow P , II \rightarrow Q , III \rightarrow T , IV \rightarrow S$
$(3)\ I \rightarrow Q , II \rightarrow S , III \rightarrow R , IV \rightarrow P$
$(4)\  I \rightarrow Q , II \rightarrow P , III \rightarrow R, IV \rightarrow T$
$(2)$ The length of the string $1,2,3$ and $4$ are kept fixed at $L _0, \frac{3 L _0}{2}, \frac{5 L _0}{4}$ and $\frac{7 L _0}{4}$, respectively. Strings $1,2,3$ and $4$ are vibrated at their $1^{\text {tt }}, 3^{\text {rd }}, 5^{\text {m }}$ and $14^{\star}$ harmonics, respectively such that all the strings have same frequency. The correct match for the tension in the four strings in the units of $T _0$ will be.
$(1)\ I \rightarrow P , II \rightarrow Q , III \rightarrow T , IV \rightarrow U$
$(2\ I \rightarrow T , II \rightarrow Q , III \rightarrow R, IV \rightarrow U$
$(3)\ I \rightarrow P , II \rightarrow Q , III \rightarrow R , IV \rightarrow T$
$(4) \  I  \rightarrow P , II \rightarrow R , III \rightarrow T , IV \rightarrow U$
  • A
    $1,2$
  • B
    $1,3$
  • C
    $1,1$
  • D
    $1,4$

Answer

For fundamental mode$($image$)$
$f =\frac{ V }{\lambda}=\frac{1}{2 L } \sqrt{\frac{ T }{\mu}}$
For string $(1)$
$f_0=\frac{1}{2 L} \sqrt{\frac{T}{\mu}} \Rightarrow(P)$
For string $(2)$
$f =\frac{1}{2 L } \sqrt{\frac{ T }{2 \mu}}=\frac{ f _0}{\sqrt{2}} \Rightarrow( R )$
For string $(3)$
$f =\frac{1}{2 L } \sqrt{\frac{ T }{3 \mu}}=\frac{ f _0}{\sqrt{3}} \Rightarrow( S )$
For string $(4)$
$f=\frac{1}{2 L} \sqrt{\frac{T}{4 \mu}}=\frac{f_0}{2} \Rightarrow(Q)$
$(2)$ For string $(1)$
Length of string $= L _0$
It is vibrating in $I ^{ ts }$ harmonic i.e. fundamental mode.
$($image$)$
$f _0=\frac{1}{2 L _0} \sqrt{\frac{ T _0}{\mu}} \Rightarrow( P )$
For string $(2)$
Length of string $=\frac{3 L _0}{2}$
It is vibrating in $III ^{\text {rd }}$ harmonic but frequency is still $f _0$.
$f_0=\frac{3 v}{2 L}$
$($image$)$
$f _0=\frac{3}{2\left(\frac{3 L _0}{2}\right)} \sqrt{\frac{ T _2}{2 \mu}}$
$\Rightarrow f _0=\frac{1}{ L _0} \sqrt{\frac{ T _2}{2 \mu}}=\frac{1}{2 L _0} \sqrt{\frac{ T _0}{\mu}}$
$\Rightarrow T _2=\frac{ T _0}{2} \Rightarrow \text { (Q) }$
For string $(3)$
Length of string $=\frac{5 L _0}{4}$
It is vibrating in $5^{\text {th }}$ harmonic but frequency is still $f _0$.
$f _0=\frac{5 V }{2 L }$
$($image$)$
$\Rightarrow f _0=\frac{5}{2\left(\frac{5 L _0}{4}\right)} \sqrt{\frac{ T _3}{3 \mu}}=\frac{1}{2 L _0} \sqrt{\frac{ T _0}{\mu}}$
$\Rightarrow \frac{2}{ L _0} \sqrt{\frac{ T _3}{3 \mu}}=\frac{1}{2 L _0} \sqrt{\frac{ T _0}{\mu}}$
$T _3=\frac{3 T _0}{16} \Rightarrow( T )$
For string $(4)$
Length of string $=\frac{7 L _0}{4}$
It is vibrating in $14^{\text {th }}$ harmonic but frequency is still $f _0$.
$($image$)$
$f _0=\frac{14 v }{2 L }$
$\Rightarrow f _0=\frac{14}{2\left(\frac{7 L _0}{4}\right)} \sqrt{\frac{ T _4}{4 \mu}}=\frac{1}{2 L _0} \sqrt{\frac{ T _0}{\mu}}$
$\Rightarrow \frac{4}{ L _0} \sqrt{\frac{ T _4}{4 \mu}}=\frac{1}{2 L _0} \sqrt{\frac{ T _0}{\mu}} \Rightarrow T _4=\frac{ T _0}{16} \Rightarrow( U )$

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