# Conceptual Questions Based on Class 11 Physics System of Particles and Rotational Motion

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Here we are providing conceptual questions for Class 11 Physics System of Particles and Rotational Motion. In this chapter, we deal with the concept of centre of mass, moment of inertia, torque and law of conservation of angular momentum.

Answers are also provided along with questions. So that students can read and understand the question. These questions are prepared by the subject experts. Students can read these conceptual questions based on System of Particles and Rotational Motion for better topic clarity and therefore will be able to score better marks in their exam.

Conceptual Questions Based on Class 11 Physics System of Particles and Rotational Motion

Conceptual questions based on Class 11 Physics Chapter 7 System of Particles and Rotational Motion is given below. Go through each and every question to understand the topic clearly.

Q.1. Should the centre of mass of a body necessarily lie inside the body?

Solution. Not necessarily. For example, the CM of a ring lies in its hollow portion.

Q.2. A body is in rotational motion. Is it necessary that a torque be acting on it?

Solution. No, torque is required only for angular acceleration.

Q.3. Is centre of mass a reality?

Solution. No. The centre of mass of a system is a hypothetical point which acts as a single mass particle of the system for an external force.

Q.4. What is the advantage of the concept of centre of mass?

Solution. Centre of mass helps us to describe the behaviour of a macroscopic body in terms of the laws developed for the microscopic bodies. If we are not concerned with the internal motion and structure of a system, the gross motion of the system can be analysed by applying Newton’s laws of motion to the CM of the system which is a point where the entire mass of the system may be assumed to be concentrated and where all the external forces are assumed to apply.

Q.5. Why do we prefer to use a wrench of longer arm?

Solution. The torque applied on the nut by the wrench is equal to the force multiplied by the perpendicular distance from the axis of rotation. Hence to increase torque a wrench of longer arm is preferred.

Q.6. A faulty balance with unequal arms has its beam horizontal. Are the weights of the two pans equal?

Solution. They are of unequal mass. Their masses are in the inverse ratio of the arms of the balance.

Q.7. Why in hand driven grinding machine, handle is put near the circumference of the stone or wheel?

Solution. For a given force, torque can be increased if the perpendicular distance of the point of application of the force from the axis of rotation is increased. Hence the handle put near the circumference produces maximum torque.

Q.8. Why are the doors provided with handles near the outer edges far away from the hinges?

Solution This makes the perpendicular distance of the line of action of force from the hinges maximum. Hence the maximum torque is exerted on the door and the door can be opened or closed easily.

Q.9. It is difficult to open the door by pushing it or pulling it at the hinges. Why?

Solution. When the force is applied at the hinges, the line of action of the force passes through the axis of rotation i.e., So we cannot open the door by pushing or pulling it at the hinges.

Q.10. A labourer standing near the top of an old wooden step ladder feels unstable. Why?

Solution. The ladder can rotate about the point of contact of the ladder with the ground. When the labourer is at the top of the ladder, the lever arm of the force is large. Hence the turning effect on the ladder will be large.

Q.11. Can a body in equilibrium while in motion? If yes, give an example.

Solution. Yes. A body in motion will be in equilibrium if it has no linear and angular accelerations. Hence a body moving with uniform velocity along a straight line will be in equilibrium.

Q.12. Why are spokes provided in a bicycle wheel?

Solution. By connecting to the rim of wheel to the axle through the spokes, the mass of the wheel gets concentrated at its rim. This increases its moment of inertia. This ensures its uniform speed.

Q.13. Why does a girl lean towards right while carrying a bag in her left hand?

Solution. When the girl carries a bag in her left hand, her CG shifts towards left. In order to bring it in the middle (for stability of equilibrium), the girl has to lean towards her right.

Q.14. A disc is recast into a thin-walled cylinder of same radius. Which will have large moment of inertia?

Solution. Hollow cylinder will have larger moment of inertia because most of its mass is located at comparatively larger distance from the axis of rotation.

Q.15. The bottom of a ship is made heavy. Why?

Solution. The bottom of a ship is made heavy so that its centre of gravity remains low. This ensures the stability of its equilibrium.

Q.16. What is the advantage of the flywheel?

Solution. In case of a flywheel, the whole mass is practically near the rim which is situated far away from the axis of rotation. This is done to increase the moment of inertia of the wheel, thereby making the motion smooth and less jerky.

Q.17. Why is it more difficult to revolve a stone tied to a large string than a stone tied to a smaller string?

Solution. The length of the string increases the distance of rotating mass from the axis of rotation and hence moment of inertia of the system is increased. Now, Torque = Moment of inertia x Angular acceleration, thus a system with large moment of inertia requires large torque for its rotation.

Q.18. How does an ice-skater, a ballet dancer or an acrobat take advantage of the principle of conservation of angular momentum?

Solution. An ice-skater, a ballet dancer or an acrobat is able to change his angular speed during the course of the performance. When the performer stretches out his hands and legs, his moment of inertia increases and the angular speed decreases. On the other hand, when he folds his hands and the legs near his body, the moment of inertia decreases, and he is able to increase the angular speed.

Q.19. A cat is able to land on its feet after a fall. Why?

Solution. When a cat falls to ground from a height, it stretches its body along with the tail so that its moment of inertia becomes high. Since IÏ‰ is to remain constant, the value of angular speed Ï‰ decreases and therefore the cat is able to land on the ground gently.

Q.20. Why there are two propellers in a helicopter?

Solution. If there were only one propeller in the helicopter then, due to conservation of angular momentum, the helicopter itself would have turned in the opposite direction.

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