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Question 511 Mark
Which of the following functions of time represent (a) simple harmonic, (b) periodic but not simple harmonic, and (c) non-periodic motion? Give period for each case of periodic motion ($\omega$ is any positive constant):

$1+\omega\text{t}+\omega^2\text{t}^2$

Answer
The given function $1+\omega\text{t}+\omega^2\text{t}^2$ is non-periodic.
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Question 521 Mark
If the displacement is represented by $\text{x}=3\sin\omega\text{t}+4\cos\omega\text{t},$ what is the amplitude?
Answer
Phase difference between $3\sin\omega\text{t}$ and $4\cos\omega\text{t}$ is $\frac{\pi}{2}.$

$\therefore$ The amplitude is $\sqrt{3^2+4^2},$ i.e., 5 units.

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Question 531 Mark
Show that for a particle executing S.H.M, velocity and displacement have a phase difference of $\frac{\pi}{2}.$
Answer
Let the displacement equation of SHM

$\text{x}=\text{a}\cos\omega\text{t}$

Velocity $\text{v}=\frac{\text{dx}}{\text{dt}}=\text{a}\omega(-\sin\omega\text{t})=-\text{a}\omega\sin\omega\text{t}$

$\Rightarrow\text{v}=\text{a}\omega\cos\Big(\frac{\pi}{2}+\omega\text{t}\Big)$

Now, phase of displacement $\phi_1=\omega\text{t}$

Phase of velocity $\phi_2=\frac{\pi}{2}+\omega\text{t}$

$\therefore$ Difference in phase of velocity to that of phase of displacement

$\triangle\phi=\phi_2-\phi_1=\Big(\frac{\pi}{2}+\omega\text{t}\Big)-(\omega\text{t})=\frac{\pi}{2}$

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Question 541 Mark
If the total energy with an oscillating system is E, what is the kinetic energy at $\text{x}=\frac{\text{A}}{3}?$
Answer
K.E. $=\frac{1}{2}\text{m}\omega^2(\text{A}^2-\text{x}^2)$

Total energy, $\text{E}=\frac{1}{2}\text{m}\omega^2\text{A}^2$

At, $\text{x}=\frac{\text{A}}{3},$

K.E. $=\frac{1}{2}\text{m}\omega^2\Big(\text{A}^2-\frac{\text{A}^2}{9}\Big)$

$=\frac{8}{9}\frac{1}{2}\text{m}\omega^2\text{A}^2=\frac{8}{9}\text{E}$

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Question 551 Mark
A particle is vibrating in SHM when the displacements of the particle from its equilibrium position are x1 and x2, it has velocities v1 and v2 respectively. Show that its time period is given by $\text{T}=2\pi\sqrt{\frac{\text{x}_1^2-\text{x}_2^2}{\text{v}_2^2-\text{v}_1^2}}$.
Answer
The particle velocity in SHM is given by: $\text{v}=\omega\sqrt{\text{A}^2-\text{x}^2}$  where A is the amplitude of oscillation.

For displacement $\text{x}=\text{x}_1$

$\text{v}_1=\omega\sqrt{\text{A}^2-\text{x}_1^2}$

$\text{v}_1^2=\omega^2(\text{A}^2-\text{x}_1^2)\cdots\text{(i)}$

For displacement $\text{x}=\text{x}_2$

$\text{v}_2=\omega\sqrt{\text{A}^2-\text{x}_2^2}$

$\text{v}_2^2=\omega^2(\text{A}^2-\text{x}_2^2)\cdots\text{(ii)}$

Subtracting (i) from (ii), we have

$\text{v}_2^2=\text{v}_2\omega^2(\text{x}_1^2-\text{x}_2^2)$

$\Rightarrow\omega=\sqrt{\frac{(\text{v}_2^2-\text{v}_1^2)}{(\text{x}_1^2-\text{x}_2^2)}}$

$\therefore$ Period of oscillation $\text{T}=\frac{2\pi}{\omega}$

$=2\pi\sqrt{\frac{(\text{x}_2^2-\text{x}_2^2)}{(\text{v}_1^2-\text{v}_2^2)}}$

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Question 561 Mark
The amplitude of a harmonic oscillator is doubled. How does its energy change?
Answer
As $\text{E}\propto\text{A}^2$ the energy of harmonic oscillator will became 4 times, its original value when its amplitude is doubled.
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Question 571 Mark
A simple harmonic motion is described by a = -16x where a → acceleration, x → displacement in m. What is the time period?
Answer
For S.H.M., $\text{a}=-\omega^2\text{x}$

Comparing with $\text{a}=-16\text{x}$

$\because\omega^2=16\Rightarrow\omega=\frac{2\pi}{\text{T}}=\sqrt{16}=4$

$\therefore\text{T}=\frac{\pi}{2}\text{ second}$

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Question 581 Mark
State force law for a simple harmonic motion.
Answer
Force $\text{F}\propto-\text{x}$

$\Rightarrow\text{F = kx}$

$\Rightarrow\text{F}=-\text{m}\omega^2\text{x}.$

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Question 591 Mark
A simple pendulum of length l suspended from a roof of a trolley which moves in a horizontal direction with an acceleration a. Find its time period of oscillation.
Answer
The trolley is accelerated horizontally by a. So, there will be two accelerations, g vertically down and horizontal acceleration a. The net acceleration is $\sqrt{\text{g}^2+\text{a}^2}.$ The time period.

$\text{T}=2\pi\sqrt{\frac{\text{l}}{\sqrt{\text{g}^2+\text{a}^2}}}.$

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Question 601 Mark
What will be the change in the time period of a loaded spring when taken to moon?
Answer
No change, since $\text{T}=2\pi\sqrt{\frac{\text{m}}{\text{k}}}$
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Question 611 Mark
A cylindrical wooden block of cross-section 15.0cm2 and mass 230gm is floated over water with an extra weight 50gm attached to its bottom. The cylinder floats vertically. From the state of equilibrium, it is slightly depressed and released. If the specific gravity of wood is 0.30 and g = 9.8m per sec2, find the frequency of oscillation of the block.
Answer
Area of cross-section of the block $=\pi\text{r}^2=15\text{cm}^2$

= 15 × 10-4m2

Total weight of the block = (230 + 50) = 280gm

= 0.28kg

Density of wood = 0.30gm/ c.c

= 300kg/ m3 

Density of water = 103kg

When the cylinder is depressed in water through a distance y the restoring force = weight of water displaced

F = Aydg = (15 × 10-4) × 103 × 9.8 newton/ metre

= 1.5 × 9.8 N/ m

Hence the frequenry of orillation is given by

$=\frac{1}{2\pi}\sqrt{\Big(\frac{\text{k}}{\text{m}}\Big)}$

$=\frac{1}{2\pi}\sqrt{\frac{1.5\times9.8}{0.28}}$

$=1.15\text{Hz}$

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Question 621 Mark
A simple pendulum is mounted inside a space craft. What should be its time period of oscillation?
Answer
A simple pendulum is mounted inside a space craft. its time period of oscillation is given by

$\text{T}=2\pi\sqrt{\frac{\text{l}}{\text{g}}}$

Here, I = 0, so T becomes infinity.

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Question 631 Mark
Will a pendulum gain or lose time when taken to the top of a mountain?
Answer
At height as we move up 'g' decreases.
Since $\text{T}\propto\frac{1}{\sqrt{\text{g}}}$ time period increases.
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Question 641 Mark
Two springs of force constants kand k2 are joined in series. What is the force constant of the combination?
Answer
The force constant k of series combination is given by $\frac{1}{\text{k}}=\frac{1}{\text{k}_1}+\frac{1}{\text{k}_2}$$\text{k}=\frac{\text{k}_1\text{k}_2}{\text{k}_1+\text{k}_2}$.
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Question 651 Mark
How will the time period of a simple pendulum change when its length is doubled?
Answer
It becomes $\sqrt{2}$ times the original time period.
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Question 661 Mark
What happens to the time period of a simple pendulum if its length is doubled?
Answer
The time period is increased by a factor of $\sqrt{2}$.
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Question 671 Mark
What forces keep the simple pendulum in simple harmonic motion?
Answer
Restoring force $\text{mg}\sin\theta$ and proper tension maintain simple harmonic motion in simple pendulum.
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Question 681 Mark
What is a second's pendulum?
Answer
A pendulum, whose time period is 2 seconds is called a second's pendulum.
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Question 691 Mark
Every SHM is periodic motion, but every periodic motion need not to be a simple harmonic motion. Do you agree? Give an example to justify your statement.
Answer
Yes, every periodic motion need not to be SHM. e.g. the motion of the earth round the sun is a periodic motion, but not simple harmonic motion as the back and forth motion is not taking place.
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Question 701 Mark
A spring of constant k is cut into two equal parts. What is the spring constant of each part?
Answer
Each part carries a constant 2k.
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Question 711 Mark
What is the maximum value of the kinetic energy/ in the case of S.H.M.?
Answer
Maximum value of K.E. is total energy, i.e. $\frac{1}{2}\text{m}\omega^2\text{A}^2.$
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Question 721 Mark
A girl swinging suddenly stands up on the swing. What is the influence on the time period and frequency?
Answer
Girl can be considered as an extended body. As the girl stands up on the swing so, the separation ‘d’ between the point of suspension and the centre of gravity decreases.
Since time period is inversely proportional to $\sqrt{\text{d}},$ time period increases and frequency decreases.
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Question 731 Mark
A simple pendulum is transferred from earth to the surface of moon. How will its time period be affected?
Answer
As value of g on moon is less than that on earth, in accordance with the relation $\text{T}=2\pi\sqrt{\frac{\text{l}}{\text{g}}}$the time period of oscillations of a simple pendulum on moon will be greater.
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Question 741 Mark
A simple harmonic motion is described by a = -16x where a is acceleration and x is displacement in meter. What is the time-period?
Answer
For simple harmonic motion, $\text{a}=\omega^2\text{x}$

comparing with $\text{a}=-16\text{x}$

$\because\omega^2=16$

$\Rightarrow\omega=\frac{2\pi}{\text{T}}=\sqrt{16}=4$

$\text{T}=\frac{\pi}{2}\text{ sec}.$

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Question 751 Mark
Two identical springs of spring constant K are attached to a block of mass m and to fixed supports as shown in Fig. When the mass is displaced from equilllibrium position by a distance x towards right, find the restoring force.

Answer
When mass is displaced from equilibrium position by a distance x towards right, the right spring gets compressed by x developing a restoring force kx towards left on the block. The left spring is stretched by an amount x developing a restoring force kx left on the block.

Developing a restoring force Kx towards

Left on the block.

F= -Kx (for left spring) and

F= -Kx (for right spring)

Restoring force, F = F1 + F2 =-2Kx

$\therefore$ F = 2Kx towards left.

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Question 771 Mark
What is the frequency of variation of potential or kinetic energy when the frequency of the oscillation is f?
Answer
Since K.E. or P.E. $\propto\cos^2\omega\text{t}$ or $\sin^2\omega\text{t},$ the frequency of variation is 2f.
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Question 781 Mark
Is it correct to say the “linear combinations of S.H.M. is a S.H.M."?
Answer
Yes, e.g., $\text{a}\sin\omega\text{t + b}\cos\omega\text{t}$ is a linear combination which is also S.H.M. with amplitude $\sqrt{\text{a}^2+\text{b}^2}.$
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Question 791 Mark
In the arrangement shown in the figure, the block of mass m is displaced, what is the frequency of oscillation?

Answer
Since extension is ofequal amount acting in the springs, the frequeocy is

$\text{f}=\frac{1}{2\pi}\sqrt{\frac{\text{k}_1+\text{k}_2}{}\text{m}}$

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Question 801 Mark
What is I second's pendulum? What is the length of a second's pendulum?
Answer
A pendulum having a time period of 2 seconds is called second's pendulum.

Since $\text{T}=2\pi\sqrt{\frac{\text{l}}{\text{g}}},$

$\text{l}=\frac{\text{T}^2\text{g}}{4\pi^2}=\frac{4\text{g}}{4\pi^2}=1\text{m}$

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Question 811 Mark
A simple harmonic motion is described by a = -16x where a → acceleration and x is displacement in meter. What is the time period?
Answer
Acceleration in simple harmonic motion, a = w2x
a = -16x = -w2x ⇒ w = 4,
Time Period, T $=\frac{2\pi}{\text{w}}=\frac{2\pi}{4}=\frac{\pi}{2}$
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Question 821 Mark
Which of the following conditions is not sufficient for S.H.M. and why?
  1. Acceleration displacement.
  2. Restoring force o displacement.
Answer
Acceleration $\propto$ displacement is not sufficient since it does not refer the direction of these quantities. As you know acceleration is always against displacement.
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Question 831 Mark
Which of the following examples represent periodic motion?
An arrow released from a bow.
Answer
There is no repetition, hence not periodic.
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Question 841 Mark
What is an epoch? Name the unit in which it is measured.
Answer
The initial difference in position from mean position expressed in radians is called epoch.
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Question 851 Mark
What is the force equation of a SHM?
Answer
According to force equation of SHM, F = -kx, where, k is a constant known as force constant.
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Question 861 Mark
Which of the following examples represent periodic motion?
A swimmer completing one (return) trip from one bank of a river to the other and back.
Answer
There is no repetition of the motion as the swimmer just completes one trip hence not periodic.
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Question 871 Mark
Which of the following examples represent (nearly) simple harmonic motion and which represent periodic but not simple harmonic motion?

General vibrations of a polyatomic molecule about its equilibrium position.

Answer
A polyatomic molecule has many natural frequencies of oscillation. Its vibration is the superposition of individual simple harmonic motions of a number of different molecules. Hence, it is not simple harmonic, but periodic.
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Question 881 Mark
A pendulum is making one oscillation in every two seconds. What is the frequency of oscillation?
Answer
0.5Hz.
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Question 891 Mark
Which of the following examples represent periodic motion?
A freely suspended bar magnet displaced from its N-S direction and released.
Answer
The motion is repeated after a certain interval of time, hence periodic. In fact, the bar magnet oscillates about its mean position with a definite period of time.
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Question 901 Mark
How would the period of spring mass system change, when it is made to oscillate horizontally and then vertically?
Answer
The time period remains same in both the cases.
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Question 911 Mark
When a pendulum clock gains time, what adjustment should be made?
Answer
When a pendulum clock gains time, it means it has gone fast i.e., it makes more vibrations per day than required. This shows that the time period of oscillation has decreased. Therefore, to correct it, the length of pendulum should be properly increased.
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Question 921 Mark
When is the tension maximum in the string of a simple pendulum?
Answer
At the lower-most point or mean position.
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Question 931 Mark
Can a motion be periodic but not oscillatory? If your answer is yes, give an example and if not explain why?
Answer
Yes, e.g., circular motion is periodic but not oscillatory.
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Question 941 Mark
Which of the following examples represent (nearly) simple harmonic motion and which represent periodic but not simple harmonic motion?
The rotation of earth about its axis.
Answer
It is periodic but not simple harmonic motion because it is not to and fro about a fixed point.
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Question 951 Mark
Is oscillation of a mass suspended by a spring simple harmonic in nature?
Answer
Yes, it is if the spring is perfectly elastic.
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Question 961 Mark
All oscillatory motions are periodic and vice-versa. Is it true?
Answer
No, There are other types of periodic motions also. Circular motion and rotatory motion are periodic but non-oscillatory.
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Question 981 Mark
Can a simple pendulum vibrate at the centre of Earth?
Answer
No. This is because of zero value of g at the centre of Earth.
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Question 991 Mark
A simple pendulum is inside a space craft. What should be its time period of vibration?
Answer
Infinity or it does not oscillate.
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Question 1001 Mark
A driver wearing an electronic digital watch goes down into sea water with terminal velocity v. How will the time in the water proof watch be affected?
Answer
It will not be affected as its action is independent of gravity and buoyant force.
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1 Marks Question - Page 2 - Physics STD 11 Science Questions - Vidyadip