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Simple harmonic motion - class-IX

Description: simple harmonic motion
Number of Questions: 58
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Tags: oscillatory motion oscillation and waves physics measurements and experimentation oscillations simple harmonic motion
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In a simple harmonic motion

  1. the potential energy is always equal to the kinetic energy

  2. the potential energy is never equal to the kinetic energy

  3. the average potential energy in any time interval is equal to the average kinetic energy in that time interval

  4. the average potential energy in one time period is equal to the average kinetic energy in this period.


Correct Option: D

The force which tries to bring a body back to its mean position is called :

  1. deforming force

  2. restoring force

  3. gravitational force

  4. buoyant force


Correct Option: B
Explanation:

At the extreme position, kinetic energy is zero and we know to throw out S.H.M.

K.E+P.E = constant
So, P.E. should be maximum at the extreme position.

A particle executes $SHM$ with a time period of $16\ s$. At time $t=2\ s$, the particle crosses the mean position while at $t=4s$, its velocity is $4ms^{-1}$. The amplitude of motion in meter is:

  1. $\sqrt{2}\pi$

  2. $16\sqrt{2} \pi$

  3. $ \dfrac{32\sqrt{2}}{\pi}$

  4. $ \dfrac{4}{\pi}$


Correct Option: C
Explanation:

Let the equation of $S.H.M$ is:-


$x=a\sin\left(\dfrac{2\pi }{T}t+\phi\right)$

when $t=2s, x=0$ and $T=16s$ So,

$0=a\sin \left(\dfrac{\pi}{4}+\phi\right)$

Or $\phi=-\dfrac{\pi}{4}$

Therefore the eqn of $S.H.M$ is:-

$x=a\sin =\left(\dfrac{2\pi}{T}t-\dfrac{\pi}{4}\right)$

Now at time $t=4s, V=4m/s$

 So
$V=d\times dt=a\times \dfrac{2\pi}{T}\cos\left(\dfrac{2\pi}{T}t-\dfrac{\pi}{4}\right)$

So, $4=a\times \dfrac{2\pi}{16}\cos\left(\dfrac{\pi}{2}-\dfrac{\pi}{4}\right)$

Or, $4=a\times \dfrac{\pi}{8}\times \dfrac{1}{\sqrt{2}}$

Or $a=\dfrac{32\sqrt{2}}{\pi}$

Hence option $C$ is correct

The particle is executing S.H.M. on a line 4 cms long. If its velocity at its mean position is 12 cm/sec, its frequency in Hertz will be :

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

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

  3. $\dfrac{\pi}{3}$

  4. $\dfrac{3}{\pi}$


Correct Option: B
Explanation:

Given,


$A=4cm$


$v=12cm/s$ at $x=0$ mean position

The velocity of particle performing S.H.M is given by

$v=\omega \sqrt{A^2-x^2}$

$12=\omega \sqrt{4^2-0}$

$12=4\omega$

$\omega =2\pi f=3$

$f=\dfrac{3}{2\pi}$

The correct option is B.

An object is attached to the bottom of a light vertical spring and set vibrating. The maximum speed of the object is 15 ${ cms }^{ -1 }$ and the period is 628 milli-seconds. The amplitude of the motion in centimeters is :

  1. 3.0

  2. 2.0

  3. 1.5

  4. 1.0


Correct Option: C
Explanation:

Given,


$T=628ms=0.628s$


$v _{max}=15cm/s=0.15m/s$

The maximum speed of the object is given by

$v _{max}=A\omega=A\dfrac{2\pi}{T}$

Amplitude, $A=\dfrac{v _{max}T}{2\pi}$

$A=\dfrac{0.15\times 0.628}{2\times 3.14}=0.015 m$

$A=1.5cm$

The correct option is C.

The different equation for linear SHM of a partial of mass $2g$ is $\dfrac {d^{2}x}{dt^{2}} + 16x = 0$. Find the force constant. $[K = mw^{2}]$.

  1. $0.02\ N/m$.

  2. $0.032\ N/m$.

  3. $0.132\ N/m$.

  4. $0.232\ N/m$.


Correct Option: B

If a body mass $36 gm$ moves with S,H,M of amplitude $A=13$ and period  $T=12 sec$. At a time $t=0$ the displacement is $x=+13 cm$. The shortest time of passage from $x=+6.5$ cm to $x=-6.5$ is

  1. 4 sec

  2. 2 sec

  3. 6 sec

  4. 3 sec


Correct Option: B
Explanation:

$\begin{array}{l} m=3bg,\, A=13,T=125 \ displacementx\left( t \right) =13\sin  \left( { \frac { { 2\pi t } }{ T }  } \right)  \end{array}$

Shortest time is at maximum slope which crosses zero. It will be from $ - 6.5\,\,to\,\,6.5\,\,$ or 2 times from $0\,to\,\,6.5$
$\begin{array}{l} 6.5=13\sin  \left( { \frac { { 2\pi t } }{ { 12 } }  } \right)  \ 0.5=\sin  \left[ { \left( { \frac { \pi  }{ 6 }  } \right) t } \right]  \ t=1\, \sec   \ total\, \, time=\, 2\times 1=2 \end{array}$

A function of time given by $\left(\sin{\omega t}-\cos{\omega t}\right)$ represents

  1. simple harmonic motion

  2. non-periodic motion

  3. periodic but not simple harmonic motion

  4. oscillatory but not simple harmonic motion


Correct Option: A
Explanation:

$\begin{array}{l} \sin  \omega t-\cos  \omega t \ =\sqrt { 2 } \left[ { \frac { 1 }{ { \sqrt { 2 }  } } \sin  \omega t-\frac { 1 }{ { \sqrt { 2 }  } } \cos  \omega t } \right]  \ =\sqrt { 2 } \left[ { \sin  \omega t\times \cos  \frac { \pi  }{ 4 } -\cos  \omega t\times \sin  \frac { \pi  }{ 4 }  } \right]  \ =\sqrt { 2 } \sin  \left( { \omega t-\frac { \pi  }{ 4 }  } \right)  \ this\, \, function\, \, represents\, \, SHM\, \, as\, \, it\, \, can\, \, be\, \, written\, \, in\, \, the\, \, form: \ a\sin  \left( { \omega t+\phi  } \right)  \ its\, \, period\, \, is,\, \, \frac { { 2\pi  } }{ \omega  }  \end{array}$

Hence,
option $(A)$ is correct answer.

A particle is subjected to two simple harmonic motions along $x$ and $y$ directions according to $x=3\sin\ 100\pi t$ $y=4\sin\ 100\pi t$

  1. Motion of particle will be on ellipse travelling in clockwise direction.

  2. Motion of particle will be on a straight line with slope $4/3$

  3. Motion will be simple harmonic motion with amplitude $5$.

  4. Phase difference between two motions is $\pi/2$.


Correct Option: A

A ring whose diameter is 1 meter, oscillates simple harmonically in a vertical plane about a nail fixed at its circumference and perpendicular to plane of ring. The time period will be

  1. 1/4 sec

  2. 1/2 sec

  3. 2sec

  4. None of these


Correct Option: C

The graph between restoring force and time in case of SHM is a

  1. parabola

  2. sine curve

  3. straight line

  4. circle


Correct Option: B
Explanation:

We know that for SHM, $x=A\sin(\omega t + \theta)$ and $F=kx=kA\sin(\omega t + \theta)$
Thus it's a sine curve.

A person weighing $60\ kg$ stands on a platform which oscillates up and down at a frequency of $2\ Hz$ and amplitude $5\ cm$. The maximum and minimum apparent weights are nearly: ($g$ = 10$\ m/s^2$)

  1. $108$ kg-wt, $12$ kg-wt

  2. $108$ kg-wt, $24$ kg-wt

  3. $54$ kg-wt, $12$ kg-wt

  4. $54$ kg-wt, $24$ kg-wt


Correct Option: A
Explanation:

$a=\omega^{2}x$
So, $a _{max}=$ $\omega^{2}A$
We know that $\omega=2\pi  f$
So, $a _{max}=\dfrac{(2\pi\times 2)^{2}\times 5}{100}$
Case I:
$N-mg=ma _{max}$
$N=m(a _{max}+g)$
$=60(10+\dfrac{16\times \pi^{2}\times 5}{100})$
$=1080 $ 

$ N=108$ kg-wt

Case II:
$mg-N=ma _{max}$
or, $N=mg-ma _{max}$
$=60(10-8)$
$=120\ N=12$ kg-wt

A body of mass $0.5$ kg is performing S.H.M. with a time period $\pi /2$ seconds. If its velocity at mean position is $1$ m/s, the restoring force acts on the body at a phase angle $60^o$ from extreme position is

  1. 0.5 N

  2. 1 N

  3. 2 N

  4. 4 N


Correct Option: B
Explanation:

$T=\dfrac{ \pi}{2}$
$V _{max}=1 m/sec$
$\omega =\dfrac{2\pi}{T}$
$V=4  rad/sec$
$V _{max=}A\omega$
$A\times 4=1$
$A=\dfrac {1}{4}$
$a=\omega^{2}x$
$F= m\omega^{2}x$
$x=A   cos   60^o$
$\therefore x=\dfrac {A}{2}$
$F=0.5\times (4)^{2}\times \dfrac {1}{4}\times \dfrac{1}{2}$
$F=1N$

Assertion : If a block is in SHM, and a new constant force acts in the direction of change, the mean position may change.
Reason :In SHM only variable forces should act on the body, for example spring force.

  1. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

  2. Both Assertion and Reason are correct and Reason is not  the correct explanation for Assertion

  3. Assertion is correct and Reason is incorrect 

  4. Assertion is incorrect and Reason is correct 


Correct Option: C
Explanation:

In SHM a constant force brings no effective change in the motion but the mean position accelerates.

Sitar maestro Ravi Shankar is playing sitar on its strings, and you, as a physicist (unfortunately without musical ears!), observed the following oddities.
I. The greater the length of a vibrating string, the smaller its frequency.
II. The greater the tension in the string, the greater is the frequency.
III. The heavier the mass of the string, the smaller the frequency.
IV. The thinner the wire, the higher its frequency.
The maestro signalled the following combination as correct one.

  1. II, III and IV

  2. I, II and IV

  3. I, II and III

  4. I, II, III and IV


Correct Option: D
Explanation:

Guitar string is a standing wave as both the ends of guitar string are fixed.

$f \propto \cfrac{1}{length}$
$f \propto \sqrt{tension}$
$f \propto \cfrac{1}{\sqrt{\mu}}$, where $\mu$ is mass per unit length.
$f \propto \cfrac{1}{\text{thickness of wire}}$

Three similar oscillators, A, B, C have the same small damping constant $r$, but different natural frequencies $\omega _0 = (k/m)^{\frac{1}{2}} : 1200 Hz, 1800 Hz, 2400 Hz$. If all three are driven by the same source at $1800 Hz$, which statement is correct for the phases of the velocities of the three?

  1. $\phi _A = \phi _B = \phi _c$

  2. $\phi _A < \phi _B = 0 < \phi _c$

  3. $\phi _A > \phi _B = 0 > \phi _c$

  4. $\phi _A > \phi _B > 0 > \phi _c$


Correct Option: A
Explanation:

$tan \phi = \dfrac{\omega L- \dfrac{1}{\omega C}}{R}$

If $\omega < \omega _0 \Longrightarrow$ circuit is capacitive

$\Longrightarrow$ it leads voltage
$\Longrightarrow$ velocity leads force.
if $\omega = \omega _0 \phi = 0$
if $\omega > {\omega} _0$ velocity lags behind force.

A stretched string of one meter length, fixed at both the ends having mass of $5 \times 10^{-4}$ kg is under tension of 20 N. It is plucked at a point situated 25 cm from one end. The stretched string would vibrate with the frequency of:

  1. $400 Hz$

  2. $100 Hz$

  3. $256 Hz$

  4. $200 Hz$


Correct Option: D

Speed v of a particle moving along a straight line, when it is at a distance x from a fixed point on the line is given by $V^2=108-9x^2$(all quantities in S. I. unit). Then

  1. The motion is uniformly accelerated along the straight line

  2. The magnitude of the acceleration at a distance 3 cm from the fixed point is $0.27m/s^2$

  3. The motion is simple harmonic about $x=6$m

  4. The maximum displacement from fixed point is 4cm.


Correct Option: C
Explanation:

$V^2=108-9x^2$


for SHM

$V^2=\omega^2(A^2-X^2)$

$V^2=9(12-X^2)$

$W=3,A=2\sqrt{3}$

$v\dfrac{dv}{dx}=9(12-2X)$

$\dfrac{dV}{dX}=0$ at $X=6$

So, it will perform SHM about $X=6m$

The equation of motion of a particle of mass $1$ g is $\frac{{{d^2}x}}{{d{t^2}}} + {\pi ^2}x = 0$ where $x$ is displacement (in m) from mean position. The frequency of oscillation is ( in Hz):

  1. $\frac{1}{2}$

  2. 2

  3. $5\sqrt {10} $

  4. $\frac{1}{{5\sqrt {10} }}$


Correct Option: A

A planck with a body of mass m placed on to it starts moving straight up with the law $y=a(1-\cos{\omega t})$ where $\omega$ is displacement. Find the time dependent force:

  1. $-ma\omega^2\cos{\omega t}$

  2. $ma\omega^2\cos{\omega t}$

  3. $ma\omega^2\sin{\omega t}$

  4. $mg+ma\omega^2\cos{\omega t}$


Correct Option: D
Explanation:

Total force on the particle will be
$F=mg+m\dfrac { d^{ 2 }y }{ dt^{ 2 } } $
since $y=a(1-\cos { \omega t } )\\ \Rightarrow \dfrac { dy }{ dt } =a\omega \sin { \omega t } \\ \Rightarrow \dfrac { d^{ 2 }y }{ dt^{ 2 } } =a\omega ^{ 2 }\cos { \omega t } $
$\Rightarrow F=mg+ma\omega ^{ 2 }\cos { \omega t } $

The frequency of a seconds pendulum is equal to :

  1. 0.5 Hz

  2. 1 Hz

  3. 2 Hz

  4. 0.1 Hz


Correct Option: A
Explanation:

The frequency of seconds pendulum is $= \dfrac{1}{2} = 0.5$ HZ

The time taken to complete $20$ oscillations by a seconds pendulum is: 

  1. $20s$

  2. $50s$

  3. $40s$

  4. $5s$


Correct Option: C
Explanation:

We know that the time period of a seconds pendulum is $T=2$ sec. One second for a swing in one direction and one second for the return swing. 

Thus, time taken to complete one oscillation is $2$ sec.
Hence, time taken to complete 20 oscillations is $2\times 20=40$ sec.

The length of a second's pendulum on the surface of the earth is equal to 99.49 cm. True or false.

  1. True

  2. False


Correct Option: A
Explanation:

The time period of seconds pendulum T = 2 seconds, acceleration due to gravity at earth g= 980 $\dfrac { cm }{ { s }^{ 2 } } $,It '$l$' is the length of pendulum,

$l=\dfrac { { T }^{ 2 }g }{ 4{ \pi  }^{ 2 } } \ \Rightarrow l=\dfrac { 4\times 980 }{ 4\times \left( \dfrac { 22 }{ 7 }  \right) ^{ 2 } } =\dfrac { 4\times 980\times 49 }{ 4\times 489 } =99.49$

If R is the radius of the earth and g the acceleration due to gravity on the earth's surface, the mean density of the earth is

  1. 4πG/3gR

  2. 3πR/4gG

  3. 3g/4πRG     

  4. πRg/12G


Correct Option: C
Explanation:

We know that

$g=\cfrac{GM}{R^2}$
Also, density $=mass\times volume$
$M=density\times volume\M=P\times\cfrac{4\pi R^3}{3R^2}=P\times\cfrac{4\pi R}{3}$
Put value of m in $g=\cfrac{GM}{R^2}\P=\cfrac{3g}{4\pi RG}$

Let the time period of a seconds pendulum is $2.5\ s.$ Tell by how much time will the clock behind in $10\ hrs.$

  1. $2.5\ hr$

  2. $2\ hr$

  3. $1.5\ hr$

  4. $None$


Correct Option: C

The mass of a bob, suspended in a simple pendulum, is halved from the initial mass, its time period will :

  1. Be less

  2. Be more

  3. Remain unchanged

  4. None of these


Correct Option: C
Explanation:

The time period of simple pendulum id given by

$T=2\pi \sqrt{\dfrac{l}{g}}$
where, $l=$ length of simple pendulum
$g=$ acceleration due to gravity
$T=$ Time period
The time period of simple pendulum is independent of the mass of bob, the time period remains unchanged,when mass of bob will change.
The correct option is C. 

If the length of a seconds pendulum is increased by $2$% then what is loss and gain in a day?

  1. losses $764 \ s$

  2. losses $924 \ s$

  3. gains $236 \ s$

  4. losses $864 \ s$

  5. gains $346 \ s$


Correct Option: D
Explanation:

$T _0=2\pi\sqrt{\cfrac{l}{g}}\T^1=2\pi\sqrt{\cfrac{l+l\times2/100}{g}}\ \cfrac{T _0}{T^1}=\cfrac{\sqrt{100}}{\sqrt{102}}\ T^1=\cfrac{\sqrt{102}}{\sqrt{100}}T _0\T^1=1.0099T _0\approx  1.01T _0\Loss=(1.01-1)T _0=0.01T _0$

In one second, it looses $0.01sec$
$\Rightarrow$ Total time loose in one day$=(0.01\times24\times3600)seconds\=864seconds$

If the length of second's pendulum is increased by $2\%$, how many second will it lose per day?

  1. $923$ s

  2. $3727$ s

  3. $3642$ s

  4. $864$ s


Correct Option: D

The different equation of simple harmonic motion for a seconds pendulum is:

  1. $\dfrac{d^2 x}{dt^2} + x = 0$

  2. $\dfrac{d^2 x}{dt^2} + \pi x = 0$

  3. $\dfrac{d^2 x}{dt^2} + 4 \pi x = 0$

  4. $\dfrac{d^2 x}{dt^2} + \pi^2 x = 0$


Correct Option: A

A simple pendulum with a bob of mass m swings with an angular amplitude of ${ 60 }^{ 0 }$, when its angular displacement is ${ 30 }^{ 0 }$, the tension of string would be 

  1. $3\sqrt { 3 } mg$

  2. $\frac { 1 }{ 2 } mg(2\sqrt { 3 } -1)$

  3. $\frac { 1 }{ 2 } mg(3\sqrt { 3 } +2)$

  4. $\frac { 1 }{ 2 } mg(3-\sqrt { 2 } )$


Correct Option: B

The simple pendulum acts as second's pendulum on earth. Its time on a planet, whose mass and diameter are twice that of earth is:

  1. $\sqrt { 2 } s$

  2. $2\sqrt { 2 } s$

  3. $2s$

  4. $\dfrac { 1 }{ \sqrt { 2 } } s$


Correct Option: B
Explanation:

Time period of second's pendulum is two second.
Second's pendulum is that simple pendulum whose time period of vibration is two seconds. The bob of such pendulum while oscillating passes through the mean position after every one second.
Noe,
Time period of simple pendulum is given by
$T=2\pi \sqrt { \left( \dfrac { l }{ g }  \right)  } $
or  $T\propto \dfrac { 1 }{ \sqrt { g }  } $             ......(i)
but  $g=\dfrac { GM }{ { R }^{ 2 } } $      (on earth)
and  ${ g }^{ \prime  }=\dfrac { G\left( 2M \right)  }{ 4{ R }^{ 2 } } $     (on planet)
$=\dfrac { 1 }{ 2 } \dfrac { GM }{ { R }^{ 2 } } =\dfrac { g }{ 2 } $
Equation (i) gives
$\dfrac { { T }^{ \prime  } }{ T } =\dfrac { \sqrt { g }  }{ \sqrt { { g }^{ \prime  } }  } =\sqrt { 2 } $
or  ${ T }^{ \prime  }=\sqrt { 2 } T$
  $=\sqrt { 2 } \times 2              \left( T=2s \right) $
  $=2\sqrt { 2 } s$

The length of a second's pendulum at a place where g = 9.8m/s $\displaystyle ^{2}$ is 90.2 cm. State whether true or false.

  1. True

  2. False


Correct Option: B
Explanation:

Time period of pendulum is:

$T =2\pi \sqrt [  ]{ \cfrac { l }{ g }  } $
$l=\cfrac { T^{ 2 }g }{ 4\pi ^{ 2 } } $
$l=\cfrac { 4\times 9.8 }{ 4\times \pi ^{ 2 } } $
$l=0.993m=99.3m$
$l$= length of pendulum 
$g$= $9.8m/s$
$T$ = Time period of seconds pendulum $=2s$
So, our given statement is false.

A second's pendulum can be used as a timing device

  1. True

  2. False


Correct Option: A
Explanation:

The second's pendulum has a definite time period of 2 seconds. We can calculate the time by counting its no.of oscillation. So, it can be used as a time measuring equipment.

The length of a second pendulum at the surface of earth is $1\ m$. The length of second pendulum at the surface of moon, where $g$ is $\dfrac{1}{6} th$ that of earth's surface.

  1. $\dfrac{1}{6} m$

  2. $6 m$

  3. $\dfrac{1}{36}m$

  4. $36 m$


Correct Option: A
Explanation:

We know that the time period of a second pendulum is $2s$ .

At earth ,
                $T=2\pi \sqrt{l _{e}/g _{e}}$
                $2=2\pi\sqrt{l _{e}/g _{e}}$
or             $g _{e}=\pi^{2}l _{e}$  ...............................eq1

At moon ,
                $T=2\pi \sqrt{l _{m}/g _{m}}$
or             $2=2\pi \sqrt{l _{m}/(g _{e}/6)}$  , given   $g _{m}=g _{e}/6$
or             $2=2\pi\sqrt{6l _{m}/\pi^{2}l _{e}}$   ,   putting the value of $g _{m}$ from  eq1
or             $l _{m}=l _{e}/6$
Now , given  $l _{e}=1m$

Hence ,     $l _{m}=1/6m$

The length of the simple pendulum which ticks seconds is:

  1. $0.5$m

  2. $1$m

  3. $1.5$m

  4. $2$m


Correct Option: B
Explanation:

The time period of a simple pendulum is
$T = 2 \pi \sqrt{\dfrac{L}{g}}$
where L is the length of the pendulum.
or $ L = \dfrac{gT^2}{4 \pi^2}$
The time period of the simple pendulum which ticks seconds is $2$s.
$\therefore T = 2s$
Substituting in (i), we get


$L = \dfrac{(9.8 m s^{-2})(2s)^2}{4 \times (3.14)^2} = 1m$

A second's pendulum is mounted in a rocket. Its period of oscillation will decrease when the rocket is:

  1. moving up with uniform velocity

  2. moving up with uniform acceleration

  3. moving down with uniform acceleration

  4. moving around the earth in a geostationary orbit


Correct Option: B
Explanation:

Correct answer= B

As the rocket accelerates upwards, pseudo force acts in the opposite direction of propagation.
=> Pseudo force acts in downward direction and gets added up to gravitational force.
=> Effective gravity= gravitational force+ pseudo force
                                >Gravitational force
=>             g'        >         g       where g' = effective gravity
Since time period of oscillation of pendulum= √(L/g)
       where L= length of the pendulum
                   g= gravitational force acting on the pendulum
=> In this situation,
          time period of oscillation of pendulum=√(L/g')
Since g' > g
=>     √(L/g')      <     √(L/g)
=>  Time period of oscillation decreases

When a rigid body is suspended vertically and it oscillates with a small amplitude under the action of the force of gravity, the body is known as

  1. simple pendulum

  2. torsional pendulum

  3. compound pendulum

  4. seconds pendulum


Correct Option: C
Explanation:

When a rigid body is suspended vertically, and it oscillates with a small amplitude under the action of the force of gravity, the body is known as compound pendulum. Thus the periodic time of a compound pendulum is minimum when the distance between the point of suspension and the centre of gravity is equal to the radius of gyration of the body about its centre of gravity.

The correct option is (c)

 The amplitude of a simple pendulum, oscillating in air with a small spherical bob, decreases from $10\ cm$ to $8\ cm$ In $40$ seconds. Assuming that Stokes law is valid, and ratio of the coefficient of viscosity of air to that of carbon dioxide is $1.3$, the time In which amplitude of this pendulum will reduce from $10\ cm$ to $5\ cm$ in carbondioxide will be close to (in $5=1.601, \ln { 2 }  2=0.693$)

  1. $231\ s$

  2. $208\ s$

  3. $161\ s$

  4. $142\ s$


Correct Option: C

The time taken for 20 complete oscillations by a seconds pendulum is :

  1. 20 s

  2. 50 s

  3. 40 s

  4. 5 s


Correct Option: C
Explanation:

Time taken for 1 oscillation is 2 s
Time taken for 20 oscillations is $(2 \times 20) = 40  s$

A hollow pendulum bob filled with water has a small hole at the bottom through which water escapes at a constant rate. Which of the following statements describes the variation of the time period (T) of the pendulum as the water flows out?

  1. T decreases first and then increases.

  2. T increases first and then decreases.

  3. T increases throughout.

  4. T does not change.


Correct Option: B
Explanation:

$\displaystyle T = 2\pi \sqrt{\frac{l}{g}}$
First distance of comfrom suspension point will increase then decrease.

The frequency of a second's pendulum is

  1. 0.5 Hz

  2. 1.0 Hz

  3. 1.5 Hz

  4. none of these


Correct Option: A
Explanation:

Seconds pendulum takes 2 $sec$ to make one complete oscillation. 

Hence, frequency = $\dfrac{1}{2}$ Hz

The frequency of a second's pendulum is :

  1. $0.5\;Hz$

  2. $1.0\;Hz$

  3. $2.0\;Hz$

  4. $5.5\;Hz$


Correct Option: A
Explanation:

A seconds pendulum is a pendulum whose period is precisely two seconds

A simple pendulum with length $L$ and mass $m$ of the bob is oscillating with an amplitude $a$. 

Then the maximum tension in the string is :

  1. $mg$

  2. $mg[1+(\dfrac{a}{L})^{2}]$

  3. $mg[1+\dfrac{a}{2L}]^{2}$

  4. $mg[1+(\dfrac{a}{L})]^{2}$


Correct Option: D

There is a clock which gives correct time at $20^o$C is subjected to $40^o$C. The coefficient of linear expansion of the pendulum is $12\times 10^{-6}$ per $^oC$, how much is gain or loss in time?

  1. $10.3$ sec/day

  2. $19$ sec/day

  3. $5.5$ sec/day

  4. $6.8$ sec/day


Correct Option: A

Find the length of a simple pendulum such that its time period is $2\ s$.

  1. $99.4\ cm$

  2. $89.4\ cm$

  3. $79.4\ cm$

  4. $109.4\ cm$


Correct Option: A
Explanation:

$T\, =\, 2 \pi\,\sqrt{\displaystyle \frac{L}{g}}\, \Rightarrow\, T^2\, =\, 4 \pi^2\, \times\, \displaystyle \frac{L}{g}$


$T^2\, =\, 4\pi ^2\, \displaystyle \frac {L}{g}$
$\Rightarrow\, 2^2\, =\, 4\, \times\, 3.14\, \times\, 3.14\, \times\, \displaystyle \frac {L}{9.8}$
$\Rightarrow\, L\, =\, \displaystyle \frac {4\, \times\, 9.8}{4\, \times\, 3.14\, \times\, 3.14}\, m\, =\, 0.994\, m\, =\, 99.4\, cm$

A desktop toy pendulum swings back and forth once every $1.0 s$. How long is this pendulum?

  1. $0.25\, m$

  2. $0.50\, m$

  3. $0.15\, m$

  4. $0.30\, m$


Correct Option: A
Explanation:

$T\, =\, 2 \pi\,\sqrt{\displaystyle \frac{L}{g}}\, \Rightarrow\, T^2\, =\, 4 \pi^2\, \times\, \displaystyle \frac{L}{g}$ .. (1)


Putting $T = 1$ in eqn. (1), 


We get $1\, =\, 4 \pi^2\, \times\, \displaystyle \frac{L}{g}$ $\Rightarrow\, L\, =\, \displaystyle \frac{g}{4\, \pi^2}\, =\, \displaystyle \frac{9.8}{4\, \times\, 3.14\, \times\, 3.14}m\, \Rightarrow\, L\, =\, \displaystyle \frac{9.8}{39.44}m\, =\, 0.2484\, m\, =\, 0.25\, m$ (approx.)

You are designing a pendulum clock to have a period of $1.0\ s$. How long should the pendulum be ?

  1. $0.25\ m$

  2. $0.50\ m$

  3. $0.25\ cm$

  4. $0.25\ mm$


Correct Option: A
Explanation:

$T\, =\, 2 \pi\,\sqrt{\displaystyle \frac{L}{g}}\, \Rightarrow\, T^2\, =\, 4 \pi^2\, \times\, \displaystyle \frac{L}{g}$ .. (1)

Putting $T = 1$ in eqn. (1), 

We get $1\, =\, 4 \pi^2\, \times\, \displaystyle \frac{L}{g}$ $\Rightarrow\, L\, =\, \displaystyle \frac{g}{4\, \pi^2}\, =\, \displaystyle \frac{9.8}{4\, \times\, 3.14\, \times\, 3.14}m\, \Rightarrow\, L\, =\, \displaystyle \frac{9.8}{39.44}m\, =\, 0.2484\, m\, =\, 0.25\, m$ (approx.)

Two pendulums of lengths 121 cm and 100 cm start vibrating at the same instant. They are in the mean position and in the same phase. After how many vibrations of the shorter pendulum, the two will be in the same phase in the mean position? 

  1. 10 vibrations

  2. 11 vibrations

  3. 21 vibrations

  4. 20 vibrations


Correct Option: A
Explanation:

Two pendulums of length $121cm$ and $100cm$.


Let,
$L _1=121cm=\dfrac{121}{100}=1.21m$

$L _2=100cm=\dfrac{100}{100}=1m$

We have to find the vibrations made by the shorter pendulum, such that both will be in same phase from the reaction,

$T _1=longer\,pendulum$


$T _2=shorter\,pendulum$


$T=2\pi\sqrt{\dfrac{L}{g}}$

$T\propto \sqrt{L}$

$\dfrac{T _1}{T _2} \propto \sqrt{{L _1}{L _2}}$

$\dfrac{T _1}{T _2}\propto \sqrt{\dfrac{1.21}{1}}$

$\dfrac{T _1}{T _2}=\dfrac{1.1}{1}$

$10T _1=11T _2$

$10$ vibrations of longer pendulum= $11$ vibrations of shorter pendulum

Assertion (A): A wooden cube of side a floats in a non viscous liquid of density r. When it is slightly pressed and released, then it executes SHM
Reason (R): The net force responsible for SHM is the resultant of buoyancy force and true weight of the body.

  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:

When a wooden cube of side a floats in a non viscous liquid of density r and if it is slightly pressed and released it executes SHM because of the buoyancy force which is acting on the cube in upward direction. And hence the answer. 

A body is broken into two parts of masses $m _1$ and $m _2$ These parts are then separated by a distance r ,What is the value of $m _1/m _2$ so that the gravitational force has maximum possible value?

  1. $1 : 1$

  2. $1 : 2$

  3. $2: 1$

  4. $4 : 3$


Correct Option: A
Explanation:

Let the mass of the body is $m$

$\therefore \,{m _1} + {m _2} = m$
${F _G} = \frac{{G{m _1}{m _2}}}{{{r^2}}}$
${F _G} = \frac{{G{m _1}\left( {m - {m _1}} \right)}}{{{r^2}}}$
for ${F _G} \to \max \,\frac{{d\left( {{F _G}} \right)}}{{d{m _1}}} = 0$
$ = \frac{G}{{{r^2}}}\left( {m - 2{m _1}} \right) = 0$
$ = {m _1} = \frac{m}{2}$
${m _2} = \frac{m}{2}$
$\therefore {m _1}/{m _2} = 1:1$
Hence$,$ optin $(A)$ is correct$.$ 

A person normally weighing 60kg stands on a platform which oscillates up and down simple harmonically with a frequency $2Hz$ and an amplitude $5cm$.if a machine on the platform gives the person's weight,then consider the following statements :

  1. The maximum reading of machine will be $108$kg

  2. The maximum reading of machine will be $90kg$

  3. The minimum reading of machine will be $12kg$

  4. The minimum reading of the machine will be zero correct statements are:


Correct Option: A,C
Explanation:

Maximum a=$w^2A$
$=(4\pi)^2\times 0.05$
$=16 \pi^2\times 0.05$
$=8$
$mg=60 \Rightarrow m=\dfrac{60}{10}=6$
$W _{max}=m(g+a)$
$=6\times (18)$
$=108kg$
$W _{min}=6(g-a)
$=12kg

If a tunnel is cut at any orientation through earth, then in what time will, a ball released from one end, reach the other end (neglect the rotation of the earth) ?

  1. 84.6 minutes

  2. 42.3 minutes

  3. 8 minutes

  4. depends on orientation


Correct Option: B
Explanation:
Force and to rque 
As we know
g depth = $g\left ( 1-\frac{d}{R} \right )$
$g _{d} = g\left ( \frac{R-d}{R} \right )\because (R-d)=x$
$g _{d}=\frac{g}{R}x$
$F _{r}=-mg _{d}$
$F _{R}=\frac{-mgx}{R}\Rightarrow F = -KX$
$T = 2\pi \sqrt{\frac{m}{k}}\rightarrow $ As we know
$K = \frac{mg}{R}$
$T= 2\pi \sqrt{\frac{mR}{mg}}$
$T= 2\pi \sqrt{\frac{R}{g}}$
$T = 84.6 ,minutes $
$A\rightarrow B;t=\frac{T}{2}=\frac{84.6}{2}=43.2$

A ball is in simple harmonic motion in a tunnel through center of the earth. Magnitude of gravitational force acting on the ball of radius $ y _o $ ,when it is at a distance $x$ from mean position is :

  1. $\dfrac{GMm}{R^{3}}x$

  2. $\dfrac{GMm}{\left [ (R-y _{0})^{2})+x^{2} \right ]}$

  3. $\dfrac{GMm}{R^{3}}\left [ (R-y _{0})^{2}+x^{2} \right ]^{1/2}$

  4. $\dfrac{GMmR^{2}}{\left [ (R-y _{0})^{2}+x^{2} \right ]^{2}}$


Correct Option: C

A solid cube of side $a$ and density $\rho _{0}$ floats on the surface of a liquid of density $\rho $. If the cube is slightly pushed downward, then it oscillates simple harmonically with a period of:

  1. $\displaystyle 2\pi \sqrt{\frac{\rho _{0}}{\rho }\frac{a}{g}}$

  2. $\displaystyle 2\pi \sqrt{\frac{\rho }{\rho _{0}}\frac{a}{g}}$

  3. $\displaystyle 2\pi \sqrt{\frac{a}{\left ( 1-\frac{\rho }{\rho _{0}} \right )g}}$

  4. $\displaystyle 2\pi \sqrt{\frac{a}{\left ( 1+\frac{\rho }{\rho _{0}} \right )g}}$


Correct Option: A
Explanation:

$\displaystyle T=2\pi \sqrt{\frac{m}{k}}$
Here Mass of the cube is $m=\left ( \rho _{0} \right )\left ( a^{3} \right )$
Since the cube is pushed very slightly so the part dipped into the water is very negligible 

Spring constant for a negligible distance  $k=\rho _{1} \times $  bottom surface area of cube $\times g=\rho a^{2}g$
$\therefore $   $\displaystyle T=2\pi \sqrt{\frac{\rho _{0}a}{\rho g}}$

A ball is in simple harmonic motion in a tunnel through center of the earth. Total force that acts on the ball when it is at a distance $x$ from mean position is :

  1. $\dfrac{GMm}{R^{2}}$

  2. $\dfrac{GMm}{R^{3}}x$

  3. $\dfrac{GMm}{(R-y _{0})^{2}+x^{2}}$

  4. $\dfrac{GMmR^{2}}{\left [ (R-y _{0})^{2}+x^{2} \right ]^{2}}$


Correct Option: B
Explanation:


Let $M$ be the total mass of the earth. At any position of $x$ let the mass be $M'$.

Mass = density x volume

So,

$\dfrac { { M }^{ ' } }{ M } =\dfrac { \rho \times \dfrac { 4 }{ 3 } \pi { x }^{ 3 } }{ \rho \times \dfrac { 4 }{ 3 } \pi { R }^{ 3 } } =\dfrac { { x }^{ 3 } }{ { R }^{ 3 } } \\ \Rightarrow { M }^{ ' }=\dfrac { { x }^{ 3 } }{ { R }^{ 3 } } M$


So the force on the ball is given by 


${ F } _{ x }=\dfrac { G{ M }^{ ' }m }{ { x }^{ 2 } } =\dfrac { Gm }{ { x }^{ 2 } } \left( \dfrac { { x }^{ 3 } }{ { R }^{ 3 } } M \right) =\dfrac { G{ M }m }{ { R }^{ 3 } } x$

Suppose a tunnel is dug along a diameter of the earth. A particle is dropped from a point, a distance $h$ directly above the tunnel, the motion of the particle is

  1. Simple harmonic

  2. Parabolic

  3. Oscillatory

  4. Periodic


Correct Option: C,D
Explanation:

When a particle is dropped from a height $h$ above the centre of tunnel.
$(i)$ It will oscillate, through the earth to a height $h$ on both sides
$(ii)$ The motion of particle is periodic
$(iii)$ The motion of particle will not be $SHM$.

A small ball of density $\rho _{0}$ is released from rest from the surface of a liquid whose density varies with depth $h$ as $\rho =\dfrac{\rho _{0}}{2}(a+\beta h)$Mass of the ball is $m$.Select the most appropriate option:

  1. The particle will execute SHM.

  2. The maximum speed of the ball is $\dfrac{2-a}{\sqrt{2\beta }} $.

  3. Both (a) and (b) are correct.

  4. Both (a) and (b) are wrong.


Correct Option: A
Explanation:
$\rho = \dfrac {\rho _{o}}{2} (a + \beta h)$
$V = \dfrac{m}{\rho _{o}}$
Downward force, $F = mg - \dfrac{m}{\rho _{o}}\dfrac{\rho _{o}}{2}(a+\beta h)g$
$F = mg\left ( \left ( 1- \dfrac{a}{2} \right )- \dfrac{\beta h}{2}\right )$
So F is proportional to displacement from center position which is $mg (1 - \dfrac{a}{2})$ below surface
Therefore, the motion is SHM.
A = $mg (1 - \dfrac{a}{2})$
$k = \dfrac{mg \beta}{2}$
Also, $k = m\omega^{2}$
=> $\omega = \sqrt{\dfrac{g\beta}{2}}$
$v _{max} = \omega A = \dfrac{(2-a) \sqrt{g}}{\sqrt{2} \beta}$
Answer A

A cylindrical block of wood $(density=650 kg m^{-3})$, of base area $30 cm^2$ and height $54 cm$, floats in a liquid of density $900 kg$ $m^{-3}$. The block is depressed slightly and then released. The time period of the resulting oscillations of the block would be equal to that of a simple pendulum of length (nearly)

  1. 52 cm

  2. 26 cm

  3. 39 cm

  4. 65 cm


Correct Option: C
Explanation:

As block is floating it's weight should be equal to buoyancy force 
$\rho _{wood} V _{cylinder} g=\rho _{liquid} V _{displaced} g$
$V _{displaced} =\frac{\rho _{wood} V _{cylinder} }{\rho _{liquid}} $
$V _{displaced} =\frac{\rho _{wood} V _{cylinder} }{\rho _{liquid}} $...(i)
After displacing by small distance x, the net force on cylinder will be
$F=Buoyancy-w=\rho _{liquid}( V _{displaced} +A _{cylinder}\Delta x) g- \rho _{wood} V _{cylinder} g$
$F=\rho _{liquid}  V _{displaced} g+\rho _{liquid} A _{cylinder}\Delta x g-\rho _{wood} V _{wood} g$ the net force on cylinder becomes 
from equation (i) $\rho _{wood} V _{cylinder} g=\rho _{liquid} V _{displaced} g$
$F=\rho _{liquid} A _{cylinder}\Delta x g$
$ma=\rho _{liquid} A _{cylinder}\Delta x g$
$a=\frac{\rho _{liquid} A _{cylinder}}{m}\Delta x $
$a = \omega^2 \Delta x $
$\omega^2 =\frac{\rho _{liquid} A _{cylinder}}{\rho _{cylinder} V _{cylinder}}$
$\omega^2 =\frac{\rho _{liquid} A _{cylinder}}{\rho _{wood} A _{cylinder} h _{cylinder}}$
$\omega^2 =\frac{\rho _{liquid}}{\rho _{wood} h _{cylinder}}$
$\omega^2 =\frac{900}{650\times .54 }$
This should be equal to angular frequency of simple pendulum
$ \omega=\sqrt{\frac{g}{l}}$
$\sqrt{\frac{900}{650\times .54}}=\sqrt{\frac{g}{l}}$
$l=g\frac{650\times .54}{900}$
$l=.06\times 65$
$=39 cm$

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