The motion of a body about a fixed axis is termed as Rotational motion. All the particles constituting the body undergo circular motion about a common axis. Rotation of Earth about its own axis, motion of wheel, gears, motors, motion of the blades of a helicopter, a door swiveling on its hinges as we open or close it are some of common examples of rotational motion.

Key terms

• Rotational motion: the motion of a body about a fixed axis where all the particles constituting the body undergoes circular motion about a common axis.
• Rigid body: a body with a perfectly defined and unchanging shape.

When an extended object such as a wheel rotates about its axis, the motion cannot be analyzed by treating the object as a particle because at any given time different parts of the object have different linear velocities and linear accelerations. We can, however, analyze the motion by considering an extended object to be composed of a collection of particles, each of which has its own linear velocity and linear acceleration.

In order to simplify the analysis of rotational motion of an object, the object is assumed to be a rigid body. A rigid body is a body that can rotate with all its parts locked together and without any change in its shape. No matter how the body moves, the distance between two points in a rigid body remains constant (non deformable).

All real objects are deformable to some extent; however, our rigid-object model is useful in many situations in which deformation is negligible.