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Free, forced and damped oscillations - class-XI

Description: free, forced and damped oscillations
Number of Questions: 37
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Tags: oscillations option b: engineering physics propagation of sound waves oscillation and waves physics
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What impulse need to be given to a body of mass $m$, released from the surface of earth along a straight tunnel passsing through centre of earth, at the centre of earth, to bring it to rest(Mass of earth $M$, radius of earth R) 

  1. $m \sqrt { \dfrac { G M } { R } }$

  2. $\sqrt { \dfrac { G M m } { R } }$

  3. $m \sqrt { \dfrac { G M } {2 R } }$

  4. $zero$


Correct Option: D

A particle is suspended from a light vertical inelastic string of length 'l' from a fixed support. At its equilibrium position, it is projected horizontally with a speed $\sqrt{6gl}$. Find the ratio of tension on string, its horizontal position to that in vertically above the point of support.

  1. $2:1$

  2. $4:1$

  3. $3:1$

  4. $5:1$


Correct Option: B

The amplitude of a damped harmonic oscillator becomes halved in $\ minute$. After three minutes, the amplitude will becomes $\dfrac{1}{x}$ of initial amplitude, where $x$ is ?

  1. $8$

  2. $2$

  3. $3$

  4. $4$


Correct Option: A

A particle performing SHM is found at its equilibrium at $  t=1\ sec$ and it is found to have a speed of $0.25 \mathrm{m} / \mathrm{s}  $ at $  \mathrm{t}=2\ \mathrm{sec}  $ . If the period of oscillation is $6\ \mathrm{sec}  $. Calculate amplitude of oscillation

  1. $ \frac{3}{2 \pi} \mathrm{m} $

  2. $ \frac{3}{ \pi} \mathrm{m} $

  3. $ \frac{6}{2 \pi} \mathrm{m} $

  4. $ \frac{6}{ \pi} \mathrm{m} $


Correct Option: A

Assertion (A): In damped vibrations, amplitude of oscillation decreases
Reason (R): Damped vibrations indicate loss of energy due to air resistance

  1. Both A and R are true and R is the correct explanation of A

  2. Both A and R are true and R is not the correct explanation of A

  3. A is true and R is false

  4. A is false and R is true


Correct Option: A
Explanation:

Damped vibrations in which an oscillating system has the effect of reducing, restricting or preventing its oscillations.

A particle with restoring force proportional to displacement and resisting force proportional to velocity is subjected to a force $F \ sin \omega.$ If the amplitude of the particle is maximum for $\omega = \omega _1$ and the energy of the particle is maximum for $\omega = \omega _2$ then (where $\omega _0$ natural frequency of oscillation of particle)

  1. $\omega _1 = \omega _0 \ and \ \omega _2 \neq \omega _0$

  2. $\omega _1 = \omega _0 \ and \ \omega _2 = \omega _0$

  3. $\omega _1 \neq \omega _0 \ and \ \omega _2 =\omega _0$

  4. $\omega _1 \neq \omega _0 \ and \ \omega _2 \neq \omega _0$


Correct Option: C
Explanation:

As we know the energy of the particle is maximum at natural frequency. Since, the restoring force is proportional to displacement and resisting force is proportional to velocity. So the correct option is ${{\omega } _{0}}={{\omega } _{2}}\,\And \,{{\omega } _{1}}\ne\,{{\omega } _{0}}$

Few particles undergo damped harmonic motion. Values for the spring constant $k$ , the damping constant $b$ , and the mass $m$ are given below. Which leads to the smallest rate of loss of mechanical energy at the initial moment?

  1. $ k = 100N/m , m = 50 g, b = 8 g/s $

  2. $ k = 150 N/m , m = 50 g, b = 5 g/s $

  3. $ k = 150N/m , m = 10g, b = 8 g/s $

  4. $ k = 200N/m , m = 8g, b = 6 g/s $


Correct Option: B

A bar magnet oscillates with a frequency of$ 10 $ oscillations per minute. When another bar magnet is placed on its axis at a small distance, it oscillates at $14$ oscillations per minute. Now, the second bar magnet is turned so that poles are instantaneous, keeping the location same. The new frequency of oscillation will be 

  1. $2$ vibrations/min

  2. $4$ vibrations/min

  3. $10$ vibrations/min

  4. $14$ vibrations/min


Correct Option: A
Explanation:

$\dfrac{60}{10}= 2\pi \sqrt{\dfrac{l}{MB _H}}$
$\dfrac{60}{14}= 2\pi \sqrt{\dfrac{l}{M(MB _H)}}$
$\therefore \dfrac{7}{5} = \sqrt{B _H +B}{B _H} $ or $ B= \dfrac{24}{25}B _H$
Hence,
$\dfrac{60}{10}= 2\pi \sqrt{\dfrac{l}{M(B _H-B)}}= 2\pi \sqrt{\dfrac{l}{MB(1-24/25)}}$
$= 5\times 2\pi \sqrt{\dfrac{l}{2MB}} = 5 \times \dfrac{60}{10}= 30$
$\therefore f= \dfrac{60}{30} = 2$ vibrations/ min

The angular frequency of the damped oscillator is given by $\omega =\sqrt{\left(\frac{k}{m} -\dfrac{r^2}{4m^2}\right)}$ where k is the spring constant, m is the mass of the oscillator and r is the damping constant. If the ratio $\dfrac{r^2}{mk}$ is $8%$, the changed in time period compared to the undamped oscillator is approximately as follows:  

  1. Increases by 1%

  2. Decreases by 1%

  3. Decreases by 8%

  4. increases by 8%


Correct Option: B
Explanation:

$\omega = \sqrt {\dfrac{k}{m}-\dfrac{r^2}{4m^2}}= \sqrt{\dfrac{k}{m}}\sqrt{1-\dfrac{r^2}{4mk}}$
 $\approx \omega _o \left(1-\dfrac{r^2}{8mk}\right) \approx$ (1 - 1%)

The amplitude of a damped oscillator becomes $\left (\dfrac {1}{3}\right )rd$ in $2s$. If its amplitude after $6\ s$ in $\dfrac {1}{n}$ times the original amplitude, the value of $n$ is

  1. $3^{2}$

  2. $3\sqrt {2}$

  3. $3^{3}$

  4. $2^{3}$


Correct Option: C
Explanation:

Let original amplitude $=A$

Amplitude after 2 sec=$\dfrac{A}{3}$
Amplitude after next 2 sec=$\dfrac{A}{3}\times \dfrac{1}{3}=\dfrac{A}{9}$
Amplitude again  after 2 sec=$\dfrac{1}{3}\times \dfrac{A}{9}=\dfrac{A}{27}=\dfrac{A}{3^3}$
Here $n=3^3$

In damped oscillations, damping force is directly proportional to speed to oscilator . If amplitude becomes half of its maximum value in 1s , then after 2 s amplitude will be (intial amplitude =$A _{0}$)

  1. $\dfrac{1}{4}A _{0}$

  2. $\dfrac{1}{2}A _{0}$

  3. $\dfrac{1}{5}A _{0}$

  4. $\dfrac{1}{7}A _{0}$


Correct Option: A
Explanation:

In damped oscillations, damping force is directly proportional to speed to oscilator . If amplitude becomes half of its maximum value in 1s , then after 2 s amplitude will be

 

Amplitude is given by:

$A={{A} _{o}}{{e}^{-\alpha t}}$

Where, $A$ is amplitude at time t.

t is time

${{A} _{0}}$ is initial aplitude

$\alpha $ is constant

At t = 1s

$A=\dfrac{{{A} _{0}}}{2}$

So,

$ \dfrac{{{A} _{0}}}{2}={{A} _{0}}{{e}^{-\alpha }} $

$ {{e}^{-\alpha }}=\dfrac{1}{2} $

At t = 2s

$ A={{A} _{0}}{{e}^{-2\alpha }} $

$ A={{A} _{0}}{{(\dfrac{1}{2})}^{2}} $

$ A=\dfrac{{{A} _{0}}}{4} $

In damped oscillation mass is $1\ kg$ and spring constant $=100\ N/m$, damping coefficeint$=0.5\ kg\ s^{-1}$. If the mass displaced by $10\ cm$ from its mean position then what will be the value of its mechanical energy after $4$ seconds?

  1. $0.67\ J$

  2. $0.067\ J$

  3. $6.7\ J$

  4. $0.5\ J$


Correct Option: B
Explanation:
Given,

Mass, $m=1\,kg$

Spring constant, $k=100\,N/m^2$

Damping coefficient, $b=0.5\,kg/s$

Distance, $x=10\,cm$

Time, $t=4\,s$

We know,

The energy for damped oscillation, $E=\dfrac 12kx^2 e^{-\dfrac{bt}{m}}$

$E=\dfrac 12\times 100\times 0.01\times e^{-\dfrac{0.5\times 4}{1}}$

$E=\dfrac{e^{-2}}{2}=0.067\,J$

Hence the mechanical energy is $0.067\,J$

The amplitude of a damped harmonic oscillator becomes $\left (\dfrac {1}{27}\right )^{th}$ of its initial value $A _{0}$ after $6$ minute. What was the amplitude after $2\ minutes$?

  1. $A _{0}/6$

  2. $A _{0}/9$

  3. $A _{0}/4$

  4. $A _{0}/3$


Correct Option: D

The amplitude of a damped oscillator decreases to $0.9$ times its initial value in $5$ seconds. By how many times to its initial value, energy of oscillation decreases to, in $10$ seconds?

  1. $0.81$

  2. $0.73$

  3. $0.95$

  4. $0.66$


Correct Option: B

In forced oscillation displacement equation is $x(t)=A\cos(\omega _{d}t+\theta)$ then amplitude $'A'$ vary with forced angular frequency $\omega _{d}$ and natural angular frequency $'\omega'$ as (b=dumping constant)

  1. $\dfrac{F}{m\omega^{2}}$

  2. $\dfrac{F}{\left{m^{2}(\omega^{2}-\omega _{d}^{2})^{2}+\omega _{d}^{2}b^{2}\right}^{1/2}}$

  3. $\dfrac{F}{m(\omega^{2}-\omega _{d}^{2})}$

  4. $\dfrac { F }{ { \left{ m\left( { \omega } _{ d }^{ 2 }{ b }^{ 2 } \right) +\left( { \omega }^{ 2 }-{ \omega } _{ d }^{ 2 } \right) \right} }^{ 1/2 } } $


Correct Option: C

In damped oscillation, the amplitude of oscillation is reduced to 1/3 of its initial value $A _0$ at the end of 100 oscillations. When the system completes 200 oscillations, its amplitude must be

  1. $\dfrac{A _0}{2}$

  2. $\dfrac{A _0}{4}$

  3. $\dfrac{A _0}{6}$

  4. $\dfrac{A _0}{9}$


Correct Option: D

If ${ \omega  } _{ 0 }$ is natural frequency of damped forced oscillation and p that of driving force, then for amplitude resonance

  1. ${ p } _{ r }={ \omega } _{ 0 }$

  2. ${ p } _{ r }<{ \omega } _{ 0 }$

  3. ${ p } _{ r }>{ \omega } _{ 0 }$

  4. None of these


Correct Option: B

A pendulum with time of 1 s is losing energy due to damping. At certain time its energy is 45 J. If after completing 15 oscillations, its energy has become 15 J, its damping constant (in $s^{-1}$) is

  1. 2

  2. $\dfrac{1}{15} ln 3$

  3. $\dfrac{1}{2}$

  4. $\dfrac{1}{30} ln 3$


Correct Option: B

The amplitude of a damped oscillator decreases to 0.9times its original magnitude in 5s. In another 10s it will decrease to $\alpha$ times its original magnitude, where $\alpha$ equals

  1. 0.7

  2. 0.81

  3. 0.729

  4. 0.6


Correct Option: C
Explanation:

$A = {A _0}{e^{ - kt}}$

$0.9{A _0} = {A _0}{e^{ - kt}}$
$ - kt = \ln \left( {0.9} \right) \Rightarrow  - 15k = 3\ln \left( {0.9} \right)$
$A = {A _0}{e^{ - 15k}} = {A _0}{e^{ - ln{{\left( {0.9} \right)}^3}}}$
$ = {\left( {0.9} \right)^3}{A _0} = 0.729{A _0}$
Hence,
option $(C)$ is correct answer.

A mass of 50 kg is suspended from a spring of stiffness 10 kN/m. It is set oscillating and it is observed that two successive oscillations have amplitudes of 10 mm and 1 mm. Determine the damping ratio.

  1. 0.315

  2. 0.328

  3. 0.344

  4. 0.353


Correct Option: C
Explanation:


For successive amplitudes $m = 1$
amplitude reduction factor

$=ln\left( \dfrac { { x } _{ 1 } }{ { x } _{ 2 } }  \right) =ln\left(

\dfrac { 10 }{ 1 }  \right) =ln10=2.3$
amplitude reduction factor $=\dfrac { 2\pi \delta m }{ \sqrt { 1-{ \delta  }^{ 2 } }  } $
$\Rightarrow \dfrac { 2\pi \delta m }{ \sqrt { 1-{ \delta  }^{ 2 } }  } =2.3$
squaring both sides
$\dfrac { 39.478{ \delta  }^{ 2 } }{ 1-{ \delta  }^{ 2 } } =5.29\\ \Rightarrow { \delta  }^{ 2 }=0.118\\ \Rightarrow \delta =0.344$

A simple harmonic oscillator of angular frequency $2\ rad\ s^{-1}$ is acted upon by an external force $F = \sin t\ N$. If the oscillator is at rest in its equilibrium position at $t = 0$, its position at later times is proportional to

  1. $\sin t + \dfrac {1}{2} \sin 2t$

  2. $\sin t + \dfrac {1}{2} \cos 2t$

  3. $\cos t - \dfrac {1}{2} \sin 2t$

  4. $\sin t - \dfrac {1}{2} \sin 2t$


Correct Option: D

A body of mass $\text{600 gm}$ is attached to a spring of spring constant $\text{k = 100 N/m}$ and it is performing damped oscillations.  If damping constant is $0.2$ and driving force is $F = F _{0}$  $cos(\omega t)$  where $F _{0}=20N$  Find the amplitude of oscillation at resonance. 

  1. $\text{4.1 m}$

  2. $\text{0.57 m}$

  3. $\text{7.7 m}$

  4. $\text{0.98 m}$


Correct Option: C
Explanation:

As we know that the amplitude of forced oscillation is given as

$A=\dfrac{F _0}{\sqrt{m^2(\omega^2-\omega _d^2)^2+\omega _d^2b^2}}$

Here we know that when oscillator is in resonance then,

$\omega=\omega _d$

so we have

$A=\dfrac{F _0}{\omega _d b}$

$F _0=20\,N$

$m = 600\, g$

$\omega=\sqrt{\dfrac km}$

$\omega=\sqrt{\dfrac{100}{0.6}}$

$\omega=12.9\,rad/sec$

Now we have

$A=\dfrac{20}{12.9\times 0.2}$

$A=7.7\,m$

An ideal gas enclosed in a vertical cylindrical container supports a freely moving piston of mass M. The piston and the cylinder have equal cross sectional area A. When the piston is in equilibrium, the volume of the gas $ \mathrm{V} _{0}  $ and its pressure is $  \mathrm{P} _{0} $ The piston is slightly displaced from the equilibrium position and released. Assuming that the system is completely isolated from its surrounding, the piston executes a simple harmonic motion with frequency.

  1. $ \dfrac{1}{2 \pi} \dfrac{\mathrm{A} \gamma P _{0}}{V _{0} M} $

  2. $ \dfrac{1}{2 \pi} \dfrac{V _{0} M P _{0}}{A^{2} \gamma} $

  3. $ \dfrac{1}{2 \pi} \sqrt{\dfrac{A^{2} \gamma P _{0}}{M V _{0}}} $

  4. $ \dfrac{1}{2 \pi} \sqrt{\dfrac{M V _{0}}{A \gamma P _{0}}} $


Correct Option: C

The amplitude of a damped oscillator becomes half on one minute. The amplitude after 3 minute will be $\displaystyle\dfrac{1}{X}$ times the original, where $X$ is

  1. $2\times 3$

  2. $2^3$

  3. $3^2$

  4. $3\times 2^2$


Correct Option: B

The equation of a damped simple harmonic motion is $ m \frac {d^2x}{dt^2} + b \frac {dx}{dt} + kx=0 . $ Then the angular frequency of oscillation is:

  1. $ \omega = ( \frac {k}{m}+\frac {b}{4m})^{1/2} $

  2. $ \omega = ( \frac {k}{m}-\frac {b}{4m})^{1/2} $

  3. $ \omega = ( \frac {k}{m}+\frac {b^2}{4m})^{1/2} $

  4. $ \omega = ( \frac {k}{m}-\frac {b^2}{4m^2})^{1/2} $


Correct Option: D

The amplitude of a damped oscillator decreases to $0.9$ times to its original magnitude in $5s$. In another $10s$, it will decrease to $\alpha$ times to its original magnitude, where $\alpha$ equals.

  1. $0.7$

  2. $0.81$

  3. $0.729$

  4. $0.6$


Correct Option: C

A lightly damped oscillator with a frequency $\left( \omega  \right) $  is set in motion by harmonic driving force of frequency $\left( n \right) $. When $n\ll \omega $, then response of the oscillator is controlled by

  1. Oscillator frequency

  2. spring constant

  3. Damping coefficient

  4. Inertia of the mass


Correct Option: B,C

On account of damping , the frequency of a vibrating body

  1. remains unaffceted

  2. increases

  3. decreases

  4. changes erratically


Correct Option: C
Explanation:

Damping is caused by opposing force, which decreases the frequency.

Ans: C

In damped oscillations, the amplitude after $50$ oscillations is $0.8\;a _0$, where $a _0$ is the initial amplitude, then the amplitude after $150$ oscillations is

  1. $0.512\;a _0$

  2. $0.280\;a _0$

  3. Zero

  4. $a _0$


Correct Option: A
Explanation:

The amplitude, a, at time $t$ is given by $a=a _0\;exp(-\,\alpha t)$



$a _{50}=a _0\;exp(-\alpha\times 50T)=0.80\;a _0$



where $T$ is the period of oscillation



$a _{150}=a _0\;exp(-a\times 150T)$



$=a _0\;(0.8)^3=0.512\,a _0$

When an oscillator completes $100$ oscillations its amplitude reduces to $\displaystyle\dfrac{1}{3}$ of its initial value. What will be its amplitude when it completes $200$ oscillations?

  1. $\displaystyle\dfrac{1}{8}$

  2. $\displaystyle\dfrac{2}{3}$

  3. $\displaystyle\dfrac{1}{6}$

  4. $\displaystyle\dfrac{1}{9}$


Correct Option: D
Explanation:


Its is a damped oscillation, where amplitude of oscillation at time $t$ is given by $A = a _0e^{-\gamma t}$
where $a _0 = $ initial amplitude of oscillation
$\quad \gamma = $ damping constant
As per question, $\displaystyle\dfrac{a _0}{3} = a _0e^{-\gamma100/v}\quad                    ...(i)$
(where $v$ is the frequency of oscillation)
and $A = a _0e^{-\gamma200/v} \quad                ...(ii)$
From $(i)$; $\quad \displaystyle\dfrac{a _0}{3} = a _0e^{-\gamma\times100/v} \quad            ...(iii)$
Dividing equation $(ii)$ by $(iii)$, we have

$\quad \displaystyle\dfrac{A}{a _0(1/3)} = \displaystyle\dfrac{e^{-\gamma\times200/v}}{e^{-\gamma\times100/v}} = e^{-\gamma\times100/v} = \displaystyle\dfrac{1}{3}$

or $A = a _0\times\displaystyle\dfrac{1}{3}\times\displaystyle\dfrac{1}{3} = \displaystyle\dfrac{1}{9}a _0$


In reality, a spring won't oscillate for ever.               will                the amplitude of oscillation until eventually the system is at rest.

  1. Frictional force, increase

  2. Viscous force, decrease

  3. Frictional force, decrease

  4. Viscous force, increase


Correct Option: C
Explanation:

In reality, a spring won't oscillate forever. Frictional force will decrease the oscillation until eventually, the system is at rest.

Undamped oscillations are practically impossible because

  1. there is always loss of energy.

  2. there is no force opposing friction.

  3. energy is not conserved in such oscillations.

  4. None of these.


Correct Option: A
Explanation:

Underdamped oscillation is practical because there always be resistive force present in reality which will try to make an oscillating body to lose its energy. This loss of energy makes the motion damped motion.

Dampers are found on bridges

  1. to allow natural oscillations to occur.

  2. to prevent them from swaying due to wind.

  3. to prevent resonance of frequencies.

  4. None of these.


Correct Option: B
Explanation:

Dampers are found bridges to prevent them from swaying due to wind otherwise this motion can hamper the condition of a bridge.

If we wish to represent the equation for the position of the mass in terms of a differential equation, which one of these would be the most suitable?

  1. $ m \dfrac{d^2x}{dt^2} + b \dfrac{dx}{dt} + kx = 0$

  2. $ m \dfrac{d^2x}{dt^2} - b \dfrac{dx}{dt} + kx = 0$

  3. $ m \dfrac{d^2x}{dt^2} + b \dfrac{dx}{dt} - kx = 0$

  4. $ m \dfrac{d^2x}{dt^2} -b \dfrac{dx}{dt} - kx = 0$


Correct Option: A
Explanation:

The force on body oscillating in resistive medium is 

$f = -kx - bv$
$\Rightarrow m\dfrac { { d }^{ 2 }x }{ d{ t }^{ 2 } } =-kx-b\dfrac { dx }{ dt } \ \Rightarrow m\dfrac { { d }^{ 2 }x }{ d{ t }^{ 2 } } +b\dfrac { dx }{ dt } +kx=0$
k = oscillating constant 
x = displacement of body from mean position 
b = constant depends on resistive medium 
v = velocity of object = $\dfrac{dx}{dt}$
m = mass of object .

Two point masses $m _1$ and $m _2$ are coupled by a spring of spring. Constant $k$ and uncompressed length $L _0$. The spring is fully compressed and a thread ties the masses together with negligible separation between them. The tied assembly is moving in the $+x$ direction with uniform speed $v _0$. At a time, say $t = 0$, it is passing the origin and at that instant the thread breaks. The masses, attached to the spring, start oscillating. The displacement of mass $m _1$ given by $x _1(t) = v _0 t(1 - cos \omega t)$ where $A$ is a constant. Find (i) the displacement $x _2(t)$ is $m _2$, and (ii) the relationship between $A$ and $L _0$.

  1. (i) $v _0 t + \dfrac{m _1}{2m _2}A(1 - cos \omega t)$

    (ii) $A = \left(\dfrac{m _2}{2m _1 + m _2}\right)$

  2. (i) $v _0 t + \dfrac{m _1}{m _2}A(1 - cos \omega t)$

    (ii) $A = \left(\dfrac{m _2}{m _1 + m _2}\right)$

  3. (i) $v _0 t + \dfrac{m _1}{3m _2}A(1 - cos \omega t)$

    (ii) $A = \left(\dfrac{m _2}{3m _1 + m _2}\right)$

  4. (i) $v _0 t + \dfrac{m _1}{4m _2}A(1 - cos \omega t)$

    (ii) $A = \left(\dfrac{m _2}{4m _1 + m _2}\right)$


Correct Option: B

To and fro motion of a particle about its mean position is called -

  1. frequency

  2. amplitude

  3. vibration

  4. acceleration


Correct Option: C
Explanation:

To and fro motion of a particle about mean position is called vibrational motion.

The time taken by a vibrating body to complete one vibration is called its frequency. True or false.

  1. True

  2. False


Correct Option: B
Explanation:

The time taken by vibrating body to complete one vibration is called time period. so our given statement is false.

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