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**Exercise : ** Solutions of Questions on Page Number : **358**

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Q1 :
**
**

Which of the following examples represent periodic motion?

(a) A swimmer completing one (return) trip from one bank of a river to the other and back.

(b) A freely suspended bar magnet displaced from its N-S direction and released.

(c) A hydrogen molecule rotating about its center of mass.

(d) An arrow released from a bow.

**Answer :**

**Answer:** **(b)** and **(c)**

**(a)** The
swimmer's
motion is not periodic. The motion of the swimmer between the
banks of a river is back and forth. However, it does not have a
definite period. This is because the time taken by the swimmer
during his back and forth journey may not be the same.

**(b)** The motion of a freely-suspended magnet, if displaced
from its N-S direction and released, is periodic. This is because
the magnet oscillates about its position with a definite period
of time.

**(c)** When a hydrogen molecule rotates about its centre of
mass, it comes to the same position again and again after an
equal interval of time. Such motion is periodic.

**(d)** An arrow released from a bow moves only in the forward
direction. It does not come backward. Hence, this motion is not a
periodic.

Answer needs Correction? Click Here

Q2 :
**
**

Which of the following examples represent (nearly) simple harmonic motion and which represent periodic but not simple harmonic motion?

(a) the rotation of earth about its axis.

(b) motion of an oscillating mercury column in a U-tube.

(c) motion of a ball bearing inside a smooth curved bowl, when released from a point slightly above the lower most point.

(d) general vibrations of a polyatomic molecule about its equilibrium position.

**Answer :**

**Answer: (b)** and **(c)** are SHMs

**(a)** and **(d)** are periodic, but not SHMs

**(a)** During its rotation about its axis, earth comes to the
same position again and again in equal intervals of time. Hence,
it is a periodic motion. However, this motion is not simple
harmonic. This is because earth does not have a to and fro motion
about its axis.

**(b)** An oscillating mercury column in a U-tube is simple
harmonic. This is because the mercury moves to and fro on the
same path, about the fixed position, with a certain period of
time.

**(c)** The ball moves to and fro about the lowermost point of
the bowl when released. Also, the ball comes back to its initial
position in the same period of time, again and again. Hence, its
motion is periodic as well as simple harmonic.

**(d)** 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.

Answer needs Correction? Click Here

Q3 :
**
**

Figure 14.27 depicts four *x-t* plots for linear motion of a
particle. Which of the plots represent periodic motion? What is
the period of motion (in case of periodic motion)?

(a)

(b)

(c)

(d)

**Answer :**

Q4 :
**
**

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 (Ãâ€° is any positive constant):

(a) sin *Ãâ€°**t*
- cos
*Ãâ€°**t*

(b) sin^{3} *Ãâ€°**t*

(c) 3 cos (π/4 -
2*Ãâ€°**t*)

(d) cos Ãâ€°*t* + cos
3*Ãâ€°**t* + cos
5*Ãâ€°**t*

(e) exp
(-*Ãâ€°*^{2}*t*^{2})

(f) 1 + *Ãâ€°**t* +
*Ãâ€°*^{2}*t*^{2}

**Answer :**

Q5 :
**
**

A particle is in linear simple harmonic motion between two points, A and B, 10 cm apart. Take the direction from A to B as the positive direction and give the signs of velocity, acceleration and force on the particle when it is

(a) at the end A,

(b) at the end B,

(c) at the mid-point of AB going towards A,

(d) at 2 cm away from B going towards A,

(e) at 3 cm away from A going towards B, and

(f) at 4 cm away from B going towards A.

**Answer :**

Q6 :
**
**

Which of the following relationships between the acceleration
*a* and the displacement *x* of a particle involve
simple harmonic motion?

(a) *a* = 0.7*x*

(b) *a* = -200*x*^{2}

(c) *a* = -10*x*

(d) *a* = 100*x*^{3}

**Answer :**

Q7 :
**
**

The motion of a particle executing simple harmonic motion is described by the displacement function,

*x* (*t*) = *A* cos
(Ãâ€°*t* +
Ãâ€ ).

If the initial (*t* = 0) position of the particle is 1 cm
and its initial velocity is Ãâ€° cm/s,
what are its amplitude and initial phase angle? The angular
frequency of the particle is π
s^{-1}. If instead of the cosine
function, we choose the sine function to describe the SHM: *x =
B sin* (*Ãâ€°t* +
α), what are the amplitude and
initial phase of the particle with the above initial conditions.

**Answer :**

Q8 :
**
**

A spring balance has a scale that reads from 0 to 50 kg. The length of the scale is 20 cm. A body suspended from this balance, when displaced and released, oscillates with a period of 0.6 s. What is the weight of the body?

**Answer :**

Q9 :
**
**

A spring having with a spring constant 1200 N
m^{-1} is mounted on a horizontal
table as shown in Fig. A mass of 3 kg is attached to the free end
of the spring. The mass is then pulled sideways to a distance of
2.0 cm and released.

Determine (i) the frequency of oscillations, (ii) maximum acceleration of the mass, and (iii) the maximum speed of the mass.

**Answer :**

Q10 :
**
**

In Exercise 14.9, let us take the position of mass when the
spring is unstreched as *x =* 0, and the direction from left
to right as the positive direction of *x*-axis. Give
*x* as a function of time t for the oscillating mass if at
the moment we start the stopwatch (*t* = 0), the mass is

(a) at the mean position,

(b) at the maximum stretched position, and

(c) at the maximum compressed position.

In what way do these functions for SHM differ from each other, in frequency, in amplitude or the initial phase?

**Answer :**

Q11 :
**
**

Figures 14.29 correspond to two circular motions. The radius of the circle, the period of revolution, the initial position, and the sense of revolution (i.e. clockwise or anti-clockwise) are indicated on each figure.

Obtain the corresponding simple harmonic motions of the
*x*-projection of the radius vector of the revolving
particle P, in each case.

**Answer :**

Q12 :
**
**

Plot the corresponding reference circle for each of the following
simple harmonic motions. Indicate the initial (*t* = 0)
position of the particle, the radius of the circle, and the
angular speed of the rotating particle. For simplicity, the sense
of rotation may be fixed to be anticlockwise in every case:
(*x* is in cm and *t* is in *s*).

(a) *x* = -2 sin (3*t* +
π/3)

(b) *x* = cos (π/6
- *t*)

(c) *x* = 3 sin (2π*t* +
π/4)

(d) *x* = 2 cos π*t*

**Answer :**

Q13 :
**
**

Figure 14.30 (a) shows a spring of force constant *k*
clamped rigidly at one end and a mass *m* attached to its
free end. A force *F* applied at the free end stretches the
spring. Figure 14.30 (b) shows the same spring with both ends
free and attached to a mass *m* at either end. Each end of
the spring in Fig. 14.30(b) is stretched by the same force
*F*.

(a) What is the maximum extension of the spring in the two cases?

(b) If the mass in Fig. (a) and the two masses in Fig. (b) are released, what is the period of oscillation in each case?

**Answer :**

Q14 :
**
**

The piston in the cylinder head of a locomotive has a stroke (twice the amplitude) of 1.0 m. If the piston moves with simple harmonic motion with an angular frequency of 200 rad/min, what is its maximum speed?

**Answer :**

Q15 :
**
**

The acceleration due to gravity on the surface of moon is 1.7
ms^{-2}. What is the time period of a
simple pendulum on the surface of moon if its time period on the
surface of earth is 3.5 s? (*g* on the surface of earth is
9.8 ms^{-2})

**Answer :**

Q16 :
**
**

Answer the following questions:

(a) Time period of a particle in SHM depends on the force
constant *k* and mass *m* of the particle:

. A simple pendulum executes SHM approximately. Why then is the time period of a pendulum independent of the mass of the pendulum?

(b) The motion of a simple pendulum is approximately simple
harmonic for small angle oscillations. For larger angles of
oscillation, a more involved analysis shows that *T* is
greater than. Think of a
qualitative argument to appreciate this result.

(c) A man with a wristwatch on his hand falls from the top of a tower. Does the watch give correct time during the free fall?

(d) What is the frequency of oscillation of a simple pendulum mounted in a cabinthat is freely falling under gravity?

**Answer :**

Q17 :
**
**

A simple pendulum of length *l* and having a bob of mass
*M* is suspended in a car. The car is moving on a circular
track of radius *R* with a uniform speed *v*. If the
pendulum makes small oscillations in a radial direction about its
equilibrium position, what will be its time period?

**Answer :**

Q18 :
**
**

Cylindrical piece of cork of density of base area *A* and
height *h* floats in a liquid of density. The cork is depressed
slightly and then released. Show that the cork oscillates up and
down simple harmonically with a period

where *ÃÂ* is the density of cork. (Ignore damping
due to viscosity of the liquid).

**Answer :**

Q19 :
**
**

One end of a U-tube containing mercury is connected to a suction pump and the other end to atmosphere. A small pressure difference is maintained between the two columns. Show that, when the suction pump is removed, the column of mercury in the U-tube executes simple harmonic motion.

**Answer :**

Q20 :
**
**

An air chamber of volume *V* has a neck area of cross
section *a* into which a ball of mass *m* just fits and
can move up and down without any friction (Fig.14.33). Show that
when the ball is pressed down a little and released, it executes
SHM. Obtain an expression for the time period of oscillations
assuming pressure-volume variations of air to be isothermal [see
Fig. 14.33].

**Answer :**

Q21 :
**
**

You are riding in an automobile of mass 3000 kg. Assuming that
you are examining the oscillation characteristics of its
suspension system. The suspension sags 15 cm when the entire
automobile is placed on it. Also, the amplitude of oscillation
decreases by 50% during one complete oscillation. Estimate the
values of (a) the spring constant *k* and (b) the damping
constant *b* for the spring and shock absorber system of one
wheel, assuming that each wheel supports 750 kg.

**Answer :**

Q22 :
**
**

Show that for a particle in linear SHM the average kinetic energy over a period of oscillation equals the average potential energy over the same period.

**Answer :**

Q23 :
**
**

A circular disc of mass 10 kg is suspended by a wire attached to
its centre. The wire is twisted by rotating the disc and
released. The period of torsional oscillations is found to be 1.5
s. The radius of the disc is 15 cm. Determine the torsional
spring constant of the wire. (Torsional spring constant
α is defined by the relation *J*
= -α
*ÃŽÂ¸,* where *J* is the
restoring couple and *ÃŽÂ¸* the
angle of twist).

**Answer :**

Q24 :
**
**

A body describes simple harmonic motion with amplitude of 5 cm and a period of 0.2 s. Find the acceleration and velocity of the body when the displacement is (a) 5 cm, (b) 3 cm, (c) 0 cm.

**Answer :**

Q25 :
**
**

A mass attached to a spring is free to oscillate, with
angular velocity Ã”°, in a horizontal
plane without friction or damping. It is pulled to a
distance
*x*_{0}and pushed
towards the centre with a velocity
*v*_{0}at
time *t* = 0. Determine the
amplitude of the resulting oscillations in terms of the
parameters Ã”°,
*x*_{0}and
*v*_{0}. [Hint:
Start with the equation *x*
= *a* cos
(*Ã”°**t**+ÃŽÂ¸*)
and note that the initial velocity is negative.]

**Answer :**

Physics Part-2 - Physics : CBSE ** NCERT ** Exercise Solutions for Class 11th for ** Oscillations ** will be available online in PDF book form soon. The solutions are absolutely Free. Soon you will be able to download the solutions.

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