In this chapter we will study the rotational motion of a rigid body about a fixed axis. A rigid body is defined as an object that has fixed size and shape. In other words, the relative positions of its constituent particles remain constant. In actual, a rigid body does not exist – it is an useful idealization. By the term fixed axis, we mean that the axis must be fixed relative to the body and fixed in direction relative to an inertial position.
ROTATIONAL KINEMATICS
Consider a body of arbitrary shape rotating about a fixed axis ‘O’ as shown in figure(2). In a given interval all the particles lying on the line OA move to their corresponding positions lying on OB.
Although the particles of the body have different linear displacements, they all have the same Angular displacement q, which is given by
The average angular velocity of the body for a finite time interval is given by
The unit of Angular Velocity is radian per second (rad/s). The instantaneous angular velocity is defined as


It is the rate of change of the angular position q with respect to time. It is a vector quantity.
The direction of angular velocity is given by the righthand rule. We hold the right hand such that when the fingers of the right hand curve in the sense of rotation, the thumb points in the direction of ω. 

Period and Frequency of Revolution
The period T is the time for one revolution and the frequency f is the number of revolutions per second (rev/s). The period and frequency are related as
Although all particles have the same angular velocity, their speeds increase linearly with distance from the axis of rotation.
The average angular acceleration is defined as
Angular acceleration is a vector quantity measured in rad/s^{2}.
The Constant Angular Acceleration Model
When the angular acceleration is constant, we can find the change in angular velocity by integrating equation (8)
A particle moving in a circular path with speed v has a centripetal (or radial) acceleration
Table 1 Analogy Between Rotational Kinematics and Linear Kinematics
Illustration1
A disc starts rotating with constant angular acceleration of p rad/s^{2} about a fixed axis perpendicular to its plane and through its centre.
(a) Find the angular velocity of the disc after 4 s.
(b) Find the angular displacement of the disc after 4 s and
(c) Find number of turns accomplished by the disc in 4 s.
Solution
Illustration2
A wheel rotates with an angular acceleration given by a = 4at^{3} – 3bt^{2} , where t is the time and a and b are constants. If the wheel has initial angular speed w_{0}, write the equations for the:
(i) angular speed (ii) angular displacement.
Solution
Illustration –3
A flywheel of radius 20 cm starts from rest, and has a constant angular acceleration of 60 rad/s^{2}. Find
(a) the magnitude of the net linear acceleration of a point on the rim after 0.15 s
(b) the number of revolutions completed in 0.25 s
Solution
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