# Difference Between Circular Motion and Rotational Motion

## Main Difference – Circular Motion vs. Rotational Motion

In rigid-body dynamics, the terms circular motion and rotational motion both describe the motion of a body about a fixed point, but there is a difference between the two motions. The main difference between circular motion and rotational motion is that circular motion is a special case of rotational motion, where the distance between the body’s centre of mass and the axis of rotation remains fixed.

## What is Rotational Motion

Rotation or Rotational Motion refers to the motion about a fixed point. An axis of rotation is a line passing through this fixed point, perpendicular to the plane in which the body is moving. The term rotational motion can be used to describe the rotation around a fixed axis, precession (where the orientation of the axis of rotation changes), and nutation (where the axis of rotation wobbles).

For instance, the diagram below illustrate these terms with regards to Earth’s “spinning” motion about the axis of rotation through itself: Example of Rotational Motion – Earth’s Spin

Here, $R$ shows rotation about the axis of rotation, $P$ denotes precession and $N$ depicts nutation.

The axis of rotation could pass through the body itself or lie outside the body. If the axis passes through the body, the word spin may be used to describe the rotation. Revolution or orbit would refer to where the axis of rotation lies outside the rigid body.

When the axis of rotation passes through the body, if the axis passes through the centre of gravity of the body as well, the rotation could be described as centroidal rotation. For cases where the axis passes through the body, but not through the centre of gravity, the term noncentroidal rotation could be used.

It is important to bear in mind, however, that these terms are not strict definitions and that occasionally they may be used interchangeably.

## What is Circular Motion

Circular Motion is a special case of rotational motion, where the distance between the centre of mass of the rigid body and the axis of rotation remains fixed, with the rigid body travelling in a plane. Circular motion can be simply described as motion along the circumference of a circle. Circular motion is uniform if the object’s angular speed (and hence its tangential speed) remains the same. When the angular speed of an object in circular motion changes, the motion is referred to as non-uniform.

## Difference Between Rotation and Circular Motion

### Distance Between the Axis of Rotation and Centre of Mass

Rotational Motion: In rotational motion, the axis of rotation and centre of mass could change.

Circular Motion: In circular motion, the axis of rotation and centre of mass does not change.

### Orientation of the Axis of Rotation

Rotational Motion: In rotational motion, the orientation of the axis of rotation could change.

Circular Motion: In circular motion, the orientation of the axis of rotation does not change.

###### “Movement of a planet’s (e.g. Earth’s) axis” by User Herbye (German Wikipedia). Designed by Dr. H. Sulzer [CC BY-SA 3.0], via Wikimedia Commons 