Question

A tuning fork of frequency $$512\,Hz$$  makes $$4$$ beat/s with the vibrating string of a piano. The beat frequency decreases to $$2$$ beat/s when the tension in the piano string is slightly increased. The frequency of the piano string before increasing the tension was

A. $$510\,Hz$$
B. $$514\,Hz$$
C. $$516\,Hz$$
D. $$508\,Hz$$  
Answer :   $$508\,Hz$$
Solution :
Let $${n_p}$$ be the frequency of piano
$${\text{As}}\,\,\left( {{n_p} \propto \sqrt T } \right)$$
$${n_f} = $$  frequency of tuning fork $$= 512\,Hz$$
$$x =$$  Beat frequency = 4 beats/s, which is decreasing $$\left( {4 \to 2} \right)$$  after changing the tension of piano wire.
Also, tension of piano wire is increasing so $${n_p} \uparrow $$
$$\eqalign{ & {\text{Hence,}}\,\,{n_p} \uparrow - {n_f} = x \downarrow \to {\text{wrong}} \cr & {n_f} - {n_p} \uparrow = x \downarrow \to {\text{correct}} \cr & {n_p} = {n_f} - x = 512 - 4 = 508\,Hz \cr} $$

Releted MCQ Question on
Oscillation and Mechanical Waves >> Waves

Releted Question 1

A cylindrical tube open at both ends, has a fundamental frequency $$'f'$$ in air. The tube is dipped vertically in air. The tube is dipped vertically in water so that half of it is in water. The fundamental frequency of the air column in now

A. $$\frac{f}{2}$$
B. $$\frac{3\,f}{4}$$
C. $$f$$
D. $$2\,f$$
Releted Question 2

A wave represented by the equation $$y = a\cos \left( {k\,x - \omega t} \right)$$    is superposed with another wave to form a stationary wave such that point $$x = 0$$  is a node. The equation for the other wave is

A. $$a\sin \left( {k\,x + \omega t} \right)$$
B. $$ - a\cos \left( {k\,x - \omega t} \right)$$
C. $$ - a\cos \left( {k\,x + \omega t} \right)$$
D. $$ - a\sin \left( {k\,x - \omega t} \right)$$
Releted Question 3

An object of specific gravity $$\rho $$ is hung from a thin steel wire. The fundamental frequency for transverse standing waves in the wire is $$300\,Hz.$$  The object is immersed in water so that one half of its volume is submerged. The new fundamental frequency in $$Hz$$  is

A. $$300{\left( {\frac{{2\,\rho - 1}}{{2\,\rho }}} \right)^{\frac{1}{2}}}$$
B. $$300{\left( {\frac{{2\,\rho }}{{2\,\rho - 1}}} \right)^{\frac{1}{2}}}$$
C. $$300\left( {\frac{{2\,\rho }}{{2\,\rho - 1}}} \right)$$
D. $$300\left( {\frac{{2\,\rho - 1}}{{2\,\rho }}} \right)$$
Releted Question 4

A wave disturbance in a medium is described by $$y\left( {x,t} \right) = 0.02\cos \left( {50\,\pi t + \frac{\pi }{2}} \right)\cos \left( {10\,\pi x} \right)$$        where $$x$$ and $$y$$ are in metre and $$t$$ is in second

A. A node occurs at $$x = 0.15\,m$$
B. An antinode occurs at $$x = 0.3\,m$$
C. The speed wave is $$5\,m{s^{ - 1}}$$
D. The wave length is $$0.3\,m$$

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