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.

1 Mark Answers and 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.

2 Marks Answers and Questions

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.

3 Marks Answers and Questions

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.

4 Marks Answers and Questions

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.

5 Marks Question and Answer

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.