# Important Questions Class 11 Physics Chapter 14

## Class 11 Physics Chapter 14 Important Questions

### Important Questions for CBSE Class 11 Physics Chapter 14 – Oscillations

The study of oscillatory motion is a fundamental concept in Physics and is required for the understanding of many physical phenomena. In musical instruments, like the sitar, the guitar, or the violin, we come across vibrating strings that produce pleasing sounds. The membranes in drums and the diaphragms in telephone and speaker systems vibrate to and fro about their mean positions. The vibrations of air molecules make the propagation of sound possible. In a solid, the atoms vibrate about their equilibrium positions, with the average energy of vibrations being proportional to temperature. AC power supply gives voltage that oscillates alternately going positive and negative about the mean value (zero).

Chapter 14 of Class 11 Physics discusses more about oscillatory motion. Extramarks has provided the Important Questions for Class 11 Physics Chapter 14 for students to study and score well on questions that are most likely to be asked in the exams.

CBSE Class 11 Physics Chapter 14 Important Questions

Study Important Questions for Class 11 Physics Chapter 14 – Oscillations

Some of the important questions for Class 11 Physics Chapter 14 Oscillations are given below. Click the link to access the complete article on Chapter 14 Class 11 Physics Important Questions.

Q1. A girl sitting on a swing stands up.What effect will this have on the swing’s periodic time?

Ans: The time period T is directly proportional to the square root of the effective length of the pendulum (l). If the girl stands up, the effective length of the swing (i.e., pendulum) decreases, and thus the time period (T) also decreases.

Q.2. State some practical examples of S. H. M.

Ans: Some practical examples of S. H. M. are as follows.

1. Motion of piston in a gas-filled cylinder
2. Atoms vibrating in a crystal lattice
3. Motion of helical spring

Q.3 Why are the soldiers marching on a suspended bridge advised to go out of steps?

Ans: When the soldiers are marching on a suspended bridge, they are advised to go out of steps. This is because the frequency of the marching steps matches the natural frequency of the suspended bridge in this case. This causes resonance. As a result, the oscillation’s amplitude increases significantly, potentially leading to bridge collapse.

Q.4 What is the relation between uniform circular motion and simple harmonic motion (S.H.M)?

Ans: A uniform circular motion can be treated as two simple harmonic motions operating at right angles to each other.

Q.5 What is the minimum condition for a system to execute S.H.M?

Ans: The minimum condition for a body to execute S.H.M is to have elasticity and inertia.

Q.6 Is a simple pendulum’s motion strictly simple harmonic?

Ans: The motion of a simple pendulum is not strictly harmonic because we assume sin= , which is nearly true only if is very small.

Q.1 The bob of a simple vibrating pendulum is made of ice.. How will the period of swing change when the ice starts melting?

Ans: The period of swing of a simple pendulum will remain constant until the location of the bob’s centre of gravity after melting the ice remains fixed from the point of suspension. When the centre of gravity of an ice bob shifts upward after melting, the effective length of the pendulum decreases, and thus the time period of the swing decreases. Similarly, as the centre of gravity shifts downward, the time period lengthens.

Q.2 (a) A particle is in S.H.M. of amplitude 2 cm. At the extreme position, the force is 4N. What is the force at a mid-point, i.e., midway between the mean and extreme position?

Ans: 2N  is the force at a mid-point, i.e., midway between the mean and extreme position.

Q.2 (b) What happens to the time period of a simple pendulum if its length is doubled?

Ans: The time period is increased by a factor of 4l.

Q1. Which of the following examples represents periodic motion?

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

Ans: As the motion of the swimmer between the banks of the river is to and fro, it does not have a definite period. The time taken by the swimmer during his back-and-forth journey may not be the same. Hence, the swimmer’s motion is not periodic.

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

Ans: If a magnet is displaced from its N-S direction and released, then the motion of the freely-suspended magnet is periodic. This is because the magnet oscillates about its position over a definite period of time.

c) A hydrogen molecule rotating about its centre of mass

Ans: If we consider a hydrogen molecule rotating about its centre of mass, it is observed that it comes to the same position after an equal interval of time. This type of motion is called periodic motion.

d) An arrow released from a bow.

Ans: When an arrow is released from a bow, it can only move forward.There is no motion repeated at equal intervals of time. Therefore, this motion is not periodic.

Q1. Which of the following examples represents (nearly) simple harmonic motion, and which represents periodic but not simple harmonic motion?

a) The rotation of the earth on its axis

Ans: When the earth rotates about its axis, it comes to the same position at fixed intervals of time. Hence, it is a periodic motion. However, Earth does not have a to-and-fro motion about its axis. Hence, it is not a simple harmonic motion.

b) Motion of an oscillating mercury column in a U-tube

Ans:  In an oscillating mercury column in a U-tube, mercury moves to and from the same fixed position over a certain period of time. Hence, it is a simple harmonic motion.

c) Motion of a ball bearing inside a smooth, curved bowl, when released from a point slightly above the lowermost point

Ans: When a ball is released from a point slightly above the lowermost point, it moves to and from about the lowermost point of the bowl. Also, the ball comes back to its initial position in a fixed interval of time, again and again. As a result, this motion is periodic as well as simple harmonic.

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

Ans: A polyatomic molecule possesses many natural frequencies of oscillation. Its vibration is the superposition of individual simple harmonic motions of a number of different molecules. Thus, it is not simple harmonic, but periodic.

Q.1 A particle is in simple linear 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 midpoint 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.

Ans: Consider the figure given in the question. The two extreme positions, A and B are of an SHM. The positive direction of velocity is considered to be from A to B. The acceleration and the force along AP are positive, and BP is negative.

a) At the end A:

The particle that is executing SHM is momentarily at rest, being in its extreme position of motion at end A. Hence, its velocity is zero. Acceleration is positive as it is directed along the AP, and force is also positive as it is directed along the AP.

b) At the end, B:

Velocity is zero at the end, B. As acceleration and force are directed along BP,  they are negative.

c) At the midpoint of AB going towards A:

Along the direction towards A, at the midpoint of AB, the particle is at its mean position P and has a tendency to move along PA. Thus, velocity is positive. Both acceleration and force are zero.

d) At 2 cm away from B going towards A:

The position of a particle  2 cm away from B going towards A is at Q. At this position, it has the tendency to move along QP, which is the negative direction. Therefore, velocity, acceleration, and force are all positive.

e) At 3 cm from A, going towards B:

The position of a particle  3 cm away from A going towards B is at R. It has a tendency to move along RP, which is a positive direction. Here, velocity, and acceleration are all positive.

f) At 4 cm away from B going towards A:

The position of a particle 4 cm away from A going towards A, is at S. It has a proclivity to move in the negative direction, SA.Thus, velocity is negative, but acceleration is directed towards the mean position along SP. Hence, it is positive, and similarly, force is also positive.

Q.1 A car moves towards a hill and sounds its horn when it is at a distance of 1 km from the hill. The echo is heard after 5 seconds. If the speed of sound is 340 m/s, the speed of car is

A- 70 m/s

B- 60 m/s

C- 50 m/s

D- 40 m/s

Marks:1

Ans

The time taken by sound to reach the hill = 1000/340 seconds. Since the sound returns after 5 s, the distance travelled by car = 5v,where v=speed of car. Therefore, the time taken by the sound to return to the car  which is now at  1000 5v m from the hill = 1000 5v 340  seconds.5= 1000+(10005v) 340 v=60 ms 1

Q.2 A tuning fork when sounded together with a standard source of frequency 300 Hz produces 5 beats/sec. The tuning fork, when loaded with some wax, is again found to give 5 beats/sec with the standard fork source. The frequency of the fork is

a- 290 Hz

b- 259 Hz

c- 305 Hz

d- 310 Hz

Marks:1

Ans

The possible frequencies of fork are (300+5) Hz and (300-5) Hz. Since on loading the fork, again 5 beats/sec are heard, the original frequency of fork must be 305 Hz which reduces to 295 Hz on loading.

Q.3 A particle has simple harmonic motion represented by equation x=5sin 4t 6 ,  where x is the displacement. If the displacement of the particle is 3 units, then the velocity is

a-2 / 3

b-5/ 3

c-20

d-16

Marks:1

Ans

y=5sin 46 3=5sin 4t 6 sin 4t 6 = 3 5 cos 4t 6 = 1 sin 2 ? 4t 6 = 1 9 25 = 4 5 Velocity=dy dt=5×4cos 4t 6 =20× 4 5 =16units

Q.4 A uniform cylinder of mass m and radius r is attached to one end of the spring as shown in the fig on rough horizontal surface. If the cylinder is slightly displaced, then find the time period of the oscillation given that there is no slipping on the surface.

Marks:5

Ans

From energy equation ,

Total mechanical energy = constant

Q.5 A string is stretched between fixed points separated by 75 cm. It is observed to have resonant frequencies of 420 Hz and 315 Hz. There are no other resonant frequencies between these two. Find the lowest resonant frequency for this string?

Marks:3

Ans

For string fixed at both ends, resonant frequency is given by

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### 1. What are the topics covered under Chapter 14 NCERT Solutions for Class 11 Physics?

The topics covered under Chapter 14 NCERT Solutions for Class 11 Physics are as follows.

1. Introduction to the chapter
2. Periodic and oscillatory motions
3. Simple harmonic motion
4. Simple harmonic motion and uniform circular motion
5. Velocity and acceleration in simple harmonic motion
6. Force law for simple harmonic motion
7. Energy in simple harmonic motion
8. Some systems executing SHM
9. Damped simple harmonic motion
10. Forced oscillations and resonance

### 2. Can students rely on Extramarks' Important Questions for Class 11 Physics Chapter 14?

The Important Questions for Class 11 Physics Chapter 14 from Extramarks are intended to assist students in focusing on significant questions and concepts that may be asked in the exams. Every answer is explained in great detail to improve students’ conceptual knowledge and help them score better in their exams.

### 3. Define oscillations

Oscillation can be defined as any periodic motion of an object at a distance about the equilibrium position and repeating itself for a period of time. Examples include  Oscillation up and down of a Spring, oscillation swinging side by side of a pendulum (which is also called Simple harmonic motion), etc.

### 4. What is forced oscillation?

Forced oscillation refers to the oscillation where the body oscillates under influence of the external periodic force. An important term to remember when one learns about the forced oscillation is resonance. Resonance refers to the frequency of the external force being equal to the natural frequency of the oscillator. This frequency is also named the resonant frequency.

### 5. What is the Doppler effect?

The Doppler effect states that in the presence of a relative motion between the source where the sound originates and the listener, the frequency of the sound heard by the listener is completely different to the frequency of the sound that is emitted by the source.