Tag: standing waves

Questions Related to standing waves

The length of a stretched string is $2 m$. The tension in it and its mass are $10 N$ and $0.80 kg$ respectively. Arrange the following steps in a sequence to find the third harmonic of transverse wave that can be created in the string.
(a) Find the linear mass density ($m$) using the formula, $m$ $\displaystyle = \dfrac{mass (M)  of \ the \  string}{length (l)  of \ the \  string}$
(b) Collect the data from the problem and find the length($l$) tenstion ($T$) and mass ($M$) of the stretched string.
(c) The fundamental frequency of a stretched vibrating string is given by $n$ $=\displaystyle \dfrac{1}{2l} \sqrt{\dfrac{T}{m}}$
(d) The frequency of $2^{nd}$ overtone or $3^{rd}$ harmonic is given by $n _2\displaystyle = \dfrac{3}{2l}\sqrt{\dfrac{T}{m}}=3n$.

  1. a b c d

  2. d b c a

  3. b a c d

  4. b d c a


Correct Option: C
Explanation:

Collect the data from the problem and find the length ($l$), tension ($T$) and mass ($M$) of the stretched string (b). 

Find the linear mass density (m) using the formula, m $= m/l$  (a).
The fundamental frequency of a stretched vibrating string is given by, n $=\displaystyle \dfrac{1}{2l} \sqrt{\dfrac{T}{m}}$ (c).
The frequency of $2^{nd}$ overtone or $3^{rd}$ harmonic is given by $n _2=\displaystyle \dfrac{3}{2l} \sqrt{\dfrac{T}{m}}=3n$ (d)

All overtones are stationary wave.

  1. True

  2. False


Correct Option: B
Explanation:

All overtones are not stationary waves. Only those overtones which match the frequencies of the harmonics act as stationary waves.

All harmonics in a stringed instrument are

  1. Standing waves

  2. Progressive wave

  3. Electromagnetic waves

  4. Transverse waves


Correct Option: A
Explanation:

All harmonics in a stringed instrument are stationary or standing waves

The correct option is (a)

nth overtone and (n/2) harmonic are always equal

  1. True

  2. False


Correct Option: B
Explanation:

$n^{th}$ overtone is always equal to (n+1) harmonic.

For example :
First overtone = second overtone 
Second overtone = third overtone 

All harmonics are possible in a string fixed at one end

  1. True

  2. False


Correct Option: B
Explanation:

In a string fixed at one end, only odd harmonics are allowed

A set of 3 standing waves 5, 10 and 15 Hz are to be setup on a string fixed at one end. One of these frequencies are suppressed, while passing through it. Identify them:

  1. 5 Hz

  2. 10 Hz

  3. 15 Hz

  4. All the frequencies will pass through them


Correct Option: B
Explanation:

In a string fixed at one end, only odd harmonics are allowed and even harmonics are suppressed.

Thus the 10Hz standing wave is suppressed,
The correct option is (b)

The 3rd overtone for a string fixed at one end is 

  1. 3rd harmonic

  2. 5th harmonic

  3. 1st harmonic

  4. fundamental note


Correct Option: B
Explanation:

The  third overtone is the 5th harmonic, since odd harmonics are only allowed in  a string fixed at one end

The correct option is (b)

A medium will not support an infinite number of standing waves of continuously different wavelengths

  1. True

  2. False


Correct Option: A
Explanation:

Only certain sized waves will stand on any one medium and thus a medium can be tuned to accept only certain waves or certain vibrations

Find the number of beats produced per sec by the vibrations $x _1=A\sin (320\pi t)$ and $x _2=A\sin (326\pi t)$.

  1. 3

  2. 4

  3. 5

  4. 6


Correct Option: A
Explanation:

$X _1=Asin(320\Pi t)$

$X _2=Asin(326\Pi t)$
On comparing it with general equation.
$X=Asin(wt)$
Then, $w _1=320\Pi $
$w _1=2\Pi f$
frequency=160 Hz
Similarly,
$w _2=326\Pi $
$w _2=2\Pi f$
frequency=163 Hz
No of beats=163-160=3

In an organ pipe(may be closed or open) of $99$ cm length standing wave is setup, whose equation is given by longitudinal displacement.
$\xi =(0.1mm)\cos \dfrac{2\pi}{0.8}(y+1cm)\cos 2\pi (400)t$
where y is measured from the top of the tube in meters and t is second. Here $1$cm is the end correction.
The air column is vibrating in :

  1. First overtone

  2. Fifth harmonic

  3. Third harmonic

  4. Fundamental mode


Correct Option: B