Rotational Motion and Moment of Inertia Class 11 Notes | EduRev

Physics Class 11

JEE : Rotational Motion and Moment of Inertia Class 11 Notes | EduRev

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Rotational Motion

1. Rigid Body

Rigid body is defined as a system of particles in which distance between each pair of particles remains constant (with respect to time) that means the shape and size do not change, during the motion. Eg. Fan, Pen, Table, stone and so on.

Our body is not a rigid body, two blocks with a spring attached between them is also not a rigid body. For every pair of particles in a rigid body, there is no velocity of seperation or approach between the particles. In the figure shown velocities of A and B with respect to ground are Rotational Motion and Moment of Inertia Class 11 Notes | EduRev and Rotational Motion and Moment of Inertia Class 11 Notes | EduRevrespectively

Rotational Motion and Moment of Inertia Class 11 Notes | EduRevRotational Motion and Moment of Inertia Class 11 Notes | EduRev

If the above body Is rigid
VA cos θ1 = VB cos θ2

Note : With respect to any particle of rigid body the motion of any other particle of that rigid body is circular.

VBA = relative velocity of B with respect to A.

Rotational Motion and Moment of Inertia Class 11 Notes | EduRev

1.1. Pure Translational Motion :

A body is said to be in pure translational motion if the displacement of each particle is same during any time interval however small or large. In this motion all the particles have same Rotational Motion and Moment of Inertia Class 11 Notes | EduRev at an instant.

example. 

A box is being pushed on a horizontal surface.

Rotational Motion and Moment of Inertia Class 11 Notes | EduRev

Rotational Motion and Moment of Inertia Class 11 Notes | EduRev of any particle, Rotational Motion and Moment of Inertia Class 11 Notes | EduRev of any particle

Rotational Motion and Moment of Inertia Class 11 Notes | EduRev of any particle

For pure translational motion :-

Rotational Motion and Moment of Inertia Class 11 Notes | EduRev

Rotational Motion and Moment of Inertia Class 11 Notes | EduRev 

Where m1, m2, m3, ......... are the masses of different particles of the body having accelerations Rotational Motion and Moment of Inertia Class 11 Notes | EduRev respectively.

But acceleration of all the particles are same So, Rotational Motion and Moment of Inertia Class 11 Notes | EduRev

Rotational Motion and Moment of Inertia Class 11 Notes | EduRev

Where M = Total mass of the body

Rotational Motion and Moment of Inertia Class 11 Notes | EduRev = acceleration of any particle or of centre of mass of body

Rotational Motion and Moment of Inertia Class 11 Notes | EduRev

Where m1, m2, m3 ...... are the masses of different particles of the body having velocities Rotational Motion and Moment of Inertia Class 11 Notes | EduRev respectively

But velocities of all the particles are same so Rotational Motion and Moment of Inertia Class 11 Notes | EduRev

Rotational Motion and Moment of Inertia Class 11 Notes | EduRev

Where Rotational Motion and Moment of Inertia Class 11 Notes | EduRev = velocity of any particle or of centre of mass of the body.

Total Kinetic Energy of body = Rotational Motion and Moment of Inertia Class 11 Notes | EduRev

1.2. Pure Rotational Motion :

A body is said to be in pure rotational motion if the perpendicular distance of each particle remains constant from a fixed line or point and do not move parallel to the line, and that line is known as axis of rotation. In this motion all the particles have same Rotational Motion and Moment of Inertia Class 11 Notes | EduRev and at an instant. Eg. : - a rotating ceiling fan, arms of a clock.

For pure rotation motion :-

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