# Difference between circular motion and rotational motion

Are rotational motion and circular motion different or the same? If different then when can we say that a body is in circular motion, and when it's in rotational motion? I find several answers where it is stated that in circular motion, the axis of rotation is inside the body, while the axis of rotation in the other case, may be outside. Why is the axis outside in the case of rotational motion?

Maybe a better distinction to make would be between rotational motion and orbital motion. Even in that more generalized case (orbital could be something other than circular), the properties used to describe and analyze the motions are the same: axis of motion, angular momentum, moment of inertia, kinetic energy, torque, etc.

There is not a bold line of difference between the two, but generally, rotational motion refers to objects which are extended (not points) and spin about an axis which either within the material of the object or is not farther from the center of mass than the farthest dimension of the object.

Orbital (or "circular") refers to the motion of an object, which may or may not be spinning around an internal axis, around some point far from its center of mass and either repeats a path or nearly repeats a path (e.g., Mercury). More generally, the path doesn't even need to repeat because there are open orbital paths which astronomical objects routinely take.

One could say that rotational motion of a solid object is the orbital motion of thousands of particles, all moving about the same axis with the same angular frequency.

Don't get concerned about the distinction between rotational and orbital; it's not hard and fast. But you should be careful about using circular because that is very specific.

The main difference between these types of 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.

Rotational motion is based around the idea of rotation of a body about its center of mass. In rotational motion, the axis of rotation and centre of mass could change whereas in circular motion, the axis of rotation and centre of mass does not change.

Circular motion is a movement of an object along the circumference of a circle or rotation along a circular path and can either have a constant angular rotation rate and constant speed, or it can exist with a changing rate of rotation.

Think of one of those fairground rides where people sit on dummy horses. Now imagine the horses spin around the post that holds them to the base of the ride. Then you have both rotational motion of the horse and circular motion as the horses move around the ride.

In a circular motion, the axis of rotation is inside the body, while the axis of rotation in the other case, may be outside. Why the axis is outside in case of rotational motion.

I think you may have this the wrong way round.

Have a read of this link: Circular Versus Rotational Motion

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.

Circular motion is the motion of a body around a fixed point where the point lies outside the body.In rotational motion the point lies inside the rotating body. Circular motion is the cause of centripetal force while rotational motion is due to torque acting on the body.

Circular motion may be of different types and rotation is one of them. Look at this image:

http://cache3.asset-cache.net/gc/82126229-eastward-rotation-of-earth-on-its-axis-gettyimages.jpg?v=1&c=IWSAsset&k=2&d=%2B4PWm74ZNdpFCz3acuIcD6YaHRSEkbp5V1Ir8Wk%2FoFM%2F7a0aOuWJ2HGuUrFa%2BDfO

This is rotation, which is a particular type of circular motion. Now look at this:

http://mail.colonial.net/~hkaiter/Aaa_web_images2012/rotaterevolvesunearthmoon.png

Here we have circular motions but not rotation.

• Please don't post answers that essentially rely on links: They may "rot" and become unavailable, rendering the answer useless. Instead, reproduce all the relevant source material in your answer itself.
– Danu
Dec 20, 2015 at 16:20

in circular , the position of a body itself keep changing with time, but in rotational motion , the position of the body does not change .