ROTATIONAL PPT CLASS 11 PHYSICS CBSE BOARD

ROTATIONAL PP T

1. ROTATIONAL MOTION

2. OUESTION / ANSWER QUES 1: What is the advantage of concept of centre of mass? SOL : Centre of mass helps us to describe the behaviour of 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 a system can be analysed by applying Newton`s law of motion to the CM of the system which is a point where the entire mass of the system maybe assumed to be concentrated and where all the external forces are assumed to apply.

3. QUES 2: Should the centre of mass of a body necessarily lie inside the body? SOL: Not necessarily.for example,the CM of a ring lies in its hollow portion. QUES 3: Does the centre of mass of a solid necessarily lie within the body? If not,give an example. SOL: No.for example,the CM of L-shaped rod lies in the region outside the rod.

4. QUES 4: Is centre of mass a reality? SOL : No centre of mass is not a reality. It is an hypothetical point where the entire mass of an object is assumed to be concentrated. QUES 5: Is It correct to say that the centre of mass of a system of n particles is always given by the average position vectors of the constituent particle ? If not , when is this statement true ? SOL: No. This is true when all the particles of the system are of same mass .

5. QUES 6 : Name the physical quantity that corresponds to the moment of force . On what factors does it depend ? SOL: The moment of force is called torque . It depends on The magnitude of force . The perpendicular distance of the line of action of force from the axis of rotation . QUES 7: A body is in rotational motion . Is It necessary that a torque be acting on it ? SOL : No , torque is required only for angular acelaration .

6. QUES 8 : Why are the doors provided with handles near the outer edges far away from hinges ? SOL: 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 . QUES 9 : Why do we prefer to use a wrench of longer arm ? SOL: 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 .

7. Ques 10 : Why in hand driven grinding machine , handle is put near the circumference of the stone or wheel ? SOL: 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 . QUES 11 : It is difficult to open the door by pushing it or pulling at the hinges . Why ? SOL : When the force is applied at the hinges , the line of action of the force passes through the axis of rotation i.e , r = 0 , so r = r F sin 0 =0. So we cannot open the door by pushing or pulling it at the hinges e

8. QUES12: A man climbs a tall , old step ladder that has tendency to sway . He feels much more when standing near the top than when near the bottom . Why ? SOL: When the stands at the top of step ladder it increases its distance from the point of contact , the pint about which it can slip ( rotate ) . The torque exerted is equal of r F . Hence larger the r , more are the chances to sway away . QUES 13 : A faulty balance with unequal arms has its been horizontal . Are the weights of the two pans equal ? SOL: They are unequal mass . Their masses are in the inverse ratio of he arms of the balance .

9. QUES 14 :Why is it easier to open a tap with two fingers than one finger? SOL: The two finger creates a couple that produces a larger torque . QUES 15 : can the couple acting on a rigid body produce translator motion ? SOL : No, the couple acting on a rigid body cannot produce translator motion ;it can cause only rotatory motion as the resultant force is zero . QUES 16: Define the term angular impulse ? SOL :The angular impulse acting on a body is the change of angular momentum of the body about a given axis .

10. QUES 17 : If no external torque acts on a body was its angular velocity remain conserved ? SOL : No. Angular velocity is not conserved but angular momentum is conserved . QUES 18 : Give the significance of moment of inertia . SOL: the moment of inertia plays the same role in rotatory motion as the mass does in translator motion . It gives a measure of inertia in rotational motion

11. QUES19:Does moment of inertia of a body change with change of axis of rotation ? SOL: Yes , the moment of inertia of a body changes with change in orientation of the axis of rotation . QUES20: About which axis would a uniform cube have a minimum rotational inertia ? SOL : About a diagonal because the mass is more concentrated about a diagonal .

12. QUES 21: Does the radius of gyration depends upon the speed of rotation of body ? SOL : No, it depends only on the distribution of mass of the body . QUES 22: A disc is recast into a thin walled cylinder of same radius . Which will have large moment of inertia ? SOL: Hollow cylinder will have larger moment of Inertia because most of its located at comparatively larger distance from the axis of rotation .

13. QUES 22 : Why spokes are provided in a bicycle wheel ? SOL : By connecting to the rim of the 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 uniform speed . QUES 23 : will two spheres of equal masses , one solid and other hollow have equal moments of inertia ? Give reasons . SOL : the hollow sphere will have a greater moment of inertia because its entire mass is concentrated at the boundary of the sphere i.e .,at maximum distance from the axis of rotation .

14. QUES24: How does a ballet dancer vary her angular speed by outstretching her arms and legs ? SOL: 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 streches out his hands and legs , his moment of inertia increase and the angular speed decreases . On the other hand , when he folds his hands and legs near his body , the moment of inertia decreases and he is able to decrease his angular speed .

15. QUES25:What is the advantage of flywheel ? SOL: 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. QUES26:Which physical quantities are expressed by the following: a.The rate of change of angular momentum;and b.Moment of linear momentum? SOL: a.torque b.angular momentum

16. QUES 27 : A heavenly body (such as a comet revolves around the massive star in a highly elliptical orbit.is its angular momentum constant over the comes close to the orbit?(ignore any mass loss of the comet when it comes too close to the star). SOL:The heavenly body revoles around the massive star under the effect of gravitational force,which is purely radical.the torque exerted by such a force is zero.hence the angular momentum of the heavenly body remains constant over its entire orbit. QUES28: If no external torque acts on a body,will its angular velocity remain conserved? SOL: No.angular velocity is not conserved but angular momentum is conserved.

17. QUES29:When a labourer cuts down a tree,he makes a cut on the side facing the direction in which he wants it to fall.why ? SOL: the weight of trees exerts a torque about the point where the cut is made.this causes rotation of the tree about the cut. QUES30:Which component of linear momentum does not contribute to angular momentum. SOL:The radial component of linear momentum does not contribute to angular momentum. QUES31:The bottom of a ship is made heavy.why ? SOL:The bottom of a ship is made heavy so that centre of gravity remains low.this ensures the stability of its equilibrium.

18. QUES32:Why does a girl lean towards right while carrying a bag in her left hand? SOL:When the girl carries a bag in her left hand,her CG shifts toward left.in order to bring it in the middle (for stability of equilibrium),the girl has to lean towards her right. QUES33:Some heavy boxes are to be loaded along with some empty boxes on the cart.which noxes should be put on the cart first and why? SOL: The heavy boxes should be loaded first so that the CG of the loaded cart remains in the lowest position.this ensures stability of equilibrium.

19. QUES34:Why standing is not allowed in double decker bus.why ? ANS:When the passangers stand in the upper deck the CG of the loaded bus is raised which makes it less stable. QUES35:Why we cannot rise from a chair without bending a little forward? ANS:Our weight exerts a torque about pur feet.this makes difficult for us or rise from the chair.when we bend forward the CG of our body comes above our feet.the torque due to our weight becomes zero and we can easily rise from the chair

20. QUES36:A System is stable equilibrium.what can we say about its potential energy? ANS:The potential energy of the system is minimum. QUES37:Why is moment of inertia also called rotational inertia. SOL:The moment of inertia gives us a measure of inertia in rotational motion.so it is also called rotational inertia. QUES38:Give the physical significance of moment of inertia. SOL:The moment of inertia plays the same role in rotatory motion as the mass does in translator motion.it gives a measure of inertia in rotational motion.

21. QUES39:Does amount of inertia of a body change with the change of axis of rotation? ANS:Yes.the moment of inertia of a body changes with the change in position and orientation of the axis of rotation. QUES40:Does the moment of inertia of a rigid body change with the speed of rotation? ANS:No,because the moment of inertia depends upon the axis of rotation and the distribution of mass. QUES41:About which axis,the moment of inertia of a body is minimum? ANS:The moment of inertia of a body is minimum about an axis passing through its centre of mass.

22. QUES42:Does the radius of gyration depend upon the speed of rotation of the body? ANS:No,it depends only on the distribution of mass of body. QUES43:Two lensesof same mass and same radius are given.one is convex and other is concave.which one will have greater moment of inertia,when rotating about an axis perpendicular to the plane and passing through the centre? ANS:The concave lens will have greater moment of inertia because thr concave lens is thin at its centre and its mass is more concentrated at the outer edge.

23. QUES44:A disc is recast into a thin walled cylinder of same radius.which will have large moment of inertia? ANS:Hollow cylinder will have larder moment of inertia because most of its mass is located at comparatively larger distance from the axis of rotation. QUES45:Two solid spheres of same mass are made of metals of different densities.which of them has a larger moment of inertia about the diameter?why ? ANS:The sphere with small density will have larger radius and hence large moment of inertia.

24. QUES46 : Is the angular momentum of a system always conserved ? If not , under what condition is it conserved ? SOL:NO.angular momentum of a system is not always conserved.it is conserved only when no external torque acts on the system. QUES47:A flywheel is revolving with a condition angular velocity . A chip of its rim breaks and flies away . What will be the effect on its angular velocity ? SOL: Due to the increase in its mass , the moment of inertia of the flywheel will decrease . To conserve angular momentum , the angular velocity of the flywheel will increase .

25. OUES47: A cat is able to land on its feet after a fall . Why ? SOL: 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 / (|) Is to remain constant , the value of angular speed (|) decreases and therefore the cat is able to land on the ground gently. QUES48:Why there are two propellers in a helicopter ? SOL: If there were only one propeller in the helicopter then , dur to conversation of angular momentum , the helicopter itself would have turned in the opposite direction .

26. QUES49: Many rivers flow towards the equator . What effect does the sediment they carry to the seas have on the rotation of the earth ? SOL: The rivers carry sediments away f rom the axis of rotation of the rotation of the earth . This increases the rotational inertia of the earth . To conserve angular momentum , the rotational speed of the earth decreases . QUES50 :A body of A mass M while falling vertically downwards under gravity breaks into two parts ; a body of b of mass M/3and a body C of mass 2 M /3 .How does the centre of mass of bodies B and C taken together shift compared to that of A? SOL : The centre of mass of bodies of bodies B and C does not shift compared o body A , It continues to move vertically downwards under the effect of gravity . This is because there is no new external force acting on the system .

27. QUES51: “Newton ‘s law of motion are applicable to individual particles “. How would you explain the motion of a large body ? SOL: A large body can be considered as made of a number of mass particles and all mass particles interact with each other . The vector sum of all these internal forces to zero . Therefore for a large body . It can be replaced by a angle mass particle whose mass is supposed to be situated at its centre of mass and the newton ‘s law can applied . QUES52:What is the difference between centre of gravity and centre of mass? SOL:Centre of mass is a point at which whole of the mass of the body may be assumed to be consentrated to describe its motion as a particle centre of gravity is a point at which resultant of the gravitational forces on all particles of the body acts for bodies of normal dimensions centre of mas and centre of gravity coincide but centre of mass and centre of gravity relate to two different concepts even if the world devoid of gravitational force,the centre of mass would still have a meaning.

28. QUES52:There are 100 passangers in a stationary railway compartment.a physical fight starts between the passangers over some difference of opinion(1)will the position of CM of the compartment change?(2) will the position of CM of system(compartment +100 passangers ) change?give reason. SOL :(1)The position of the CM of the compartment will change because the passangers are external bodies for the compartment. (2)The position of CM of the system will not change as no external force is acting on it. QUES53:Torque and work are both equal to force times distance.then how do they differ?

29. SOL :(1)Work is a scaler quantity while torque is a vector quantity. (2)Work is measured as the product of the applied force and the distance moved by the body in the direction of force.torque is measured as the product of the force.torque is measured as the product of the force and its perpendicular distance from the axis of rotation. yashika