Why does rotation occur?

Why does rotation occur?


Torque, also called moment or moment of force (see the terminology below), is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist.


Torque is not necessarily the answer.

Our entire planet is rotating, but there is no significant external torque acting on it.

Rotation occurs because the constituent fibers of a body have their own motion which isn't always the same velocity for every fiber.

If all fibers of a body have identical velocities, the body is in pure translation.

If instead, some velocities differ, the body has rotation.

Rigid body rotation is the most intuitive. Rigid bodies are defined as bodies whose constituent fibers never move significant distances away from each other.

As a result, all velocities are dependent upon each other, and interestingly enough, there is an instantaneous center of zero velocity (ICV). From that ICV, the constraint for all velocity is that it is perpendicular to the line towards the ICV, and proportional to the distance from the ICV.

This is rigid body rotation. It is a mechanical coupling of all fibers of a rigid body such that they have dependent velocities.

Rotation change is caused by torque, not rotation in general.

Rotation in general is caused EITHER by pre-existing angular momentum at time of formation of the body, OR by a net torque from outside.


Bottom line: rotation is nothing more than a macroscopic description of the constituent point-objects of a body in their own motion patterns, that happen to differ from one another.


because of a torque!