If a body is pivoted at a point and the force is applied on the body at a suitable point, it rotates the body about the axis passing through the pivoted point. This is the turning effect of the force and the motion of the body is called the Rotational motion.
Rotational Motion
Rotational motion is an integral part of mechanics and the questions asked from this chapter are most difficult. So before starting to solve questions from this chapter, you should have a good hold on concepts from the topics laws of motion, Centre of mass and linear momentum. The questions which are asked from this topic involve conceptual approach rather than the formulabased approach. It is necessary for you to read the theory thoroughly before solving the questions. This article will help you to understand the concepts of rotational motion and how these concepts make complex questions when mixed with the concepts of Newton's basic laws.
Moment of Inertia, Angular Velocity, Torque, Rotation of a rigid body about a fixed axis, Angular Momentum, Conservation of Angular Momentum, Combined Translational and Rotational Motion of a Rigid Body, Uniform Pure Rolling, Instantaneous, Axis of Rotation, Accelerated Pure Rolling, Angular Impulse, Toppling
In our day to day life, we come across many objects which show rotational motion, the spinning of car wheels, rotation of washing machine agitator, rotation of the earth etc. Before you proceed further in this chapter, you should know what is a rigid body, a rigid body is made of too many particles but the distance between any two particles is always constant. If you remember the concepts of Translatory motion, we consider the rigid body as the point object in which we take the same linear displacement, linear velocity as well as same linear acceleration of all the particles on the rigid body. But in case of rotational motion OR translatory plus rotational motion the particles of the rigid body have different linear displacement, velocity, and acceleration.
For most of the students, rotational motion is often the pain in the neck, because it introduces many new terminologies like Moment of Inertia, Torque, and angular momentum etc. But it is not really difficult, because you can relate these terminologies with your previous knowledge of translatory motion. For example just like “centre of mass”, the moment of inertia is also the property of the object that is related to its mass distribution. Same goes with Torque, you can relate it with force.
In rotational motion, the particles of the rigid body follow a circular path around the rotational axis, the rotational axis could be fixed or it could be unfixed. The example for fixed axis rotational motion is the rotation of a fan, in which each particle on the blade is following a circular path around the axle of the motor of the fan. The example for an unfixed axis of rotational motion is spinning top, in the spinning top, the tip of the top is an unfixed axis around which all the particles are following a circular path.
First of all, learn how to calculate the moment of inertia (MOI) of different objects around different axes. You should also get familiar with using two theorems on MOI, i.e. Theorem of Parallel axes and Theorem of Perpendicular axes. Then only you should go towards calculating other parameters required in the problem. Rotational motion involves a mixture of concepts, hence You need to practice a lot more on this topic. For learning concepts through questions solving, students can take help from our Entrance360 experts. This platform is best for learning because learning concepts through problemsolving help in building concepts rather than solving all questions after reading a full chapter. This can create doubts while solving hence it will result in more confusion and student might get trapped.
Rotational Motion Formulas
Make a plan to prepare for the chapter and Stick to a Timetable. You can make timetable according to available time left for preparation and try to prepare according to it.
Don’t try to memorize MOI of different objects, rather first calculate it yourself and then memorize it.
Read carefully on the examples given in NCERT book. Start solving the questions only after you understood all the examples.
Give mock tests chapterwise from time to time, this will ensure you have a good hold on concepts.
Look at the solution/answer only after giving a good number of attempts.
For understanding concepts, students can consider NCERT books. But for questionsolving students should consider Understanding Physics by D. C. Pandey (Arihant Publications).
You don't have to study the whole book to understand the concept from this chapter because we will provide you exact page number and line number of these books where you will get these concepts to read.
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Chapter 21 
A pulley of radius 2 m is rotated about its axis by a force F = (20t  5t^{2}) newton (where t is measured in seconds) applied tangentially. If the moment of inertia of the pulley about its axis of rotation is 10 kg m^{2} , the number of rotations made by the pulley before its direction of motion if reversed, is
Option 1)
less than 3 
Option 2)
more than 3 but less than 6 
Option 3)
more than 6 but less than 9 
Option 4)
more than 9 
A mass m hangs with the help of a string wrapped around a pulley on a frictionless bearing. The pulley has mass m and radius R . Assuming pulley to be a perfect uniform circular disc, the acceleration of the mass m , if the string does not slip on the pulley, is
Option 1)  Option 2) 
Option 3)  Option 4) 
A thin horizontal circular disc is rotating about a vertical axis passing through its centre. An insect is at rest at a point near the rim of the disc. The insect now moves along a diameter of the disc to reach its other end. During the journey of the insect, the angular speed of the disc
Option 1)
remains unchanged 
Option 2)
continuously decreases 
Option 3)
continuously increases 
Option 4)
first increases and then decreases 