Rotation around a fixed axis is a special case of rotational motion. The fixed axis hypothesis excludes the possibility of a moving axis, and cannot describe such phenomena as nutation|wobbling or precession. According to Euler's rotation theorem, simultaneous rotation around more than one axis at the same time is impossible. If two rotations are forced at the same time, a new axis of rotation will appear.
The kinematics and dynamics (mechanics)|dynamics of rotation around a fixed axis of a rigid body are mathematically much simpler than those for Rigid body dynamics#Rigid body angular momentum|free rotation of a rigid body; they are entirely analogous to those of linear motion along a single fixed direction, which is not true for free rotation of a rigid body. The expressions for the kinetic energy of the object, and for the forces on the parts of the object, are also simpler for rotation around a fixed axis, than for general rotational motion. For these reasons, rotation around a fixed axis is typically taught in introductory physics courses after students have mastered linear motion; the full generality of rotational motion is not usually taught in introductory physics classes.
Rotation around a fixed axis is a special case of rotational motion. The fixed axis hypothesis excludes the possibility of a moving axis, and cannot describe such phenomena as nutation|wobbling or precession. According to Euler's rotation theorem, simultaneous rotation around more than one axis at the same time is impossible. If two rotations are forced at the same time, a new axis of rotation will appear.
The kinematics and dynamics (mechanics)|dynamics of rotation around a fixed axis of a rigid body are mathematically much simpler than those for Rigid body dynamics#Rigid body angular momentum|free rotation of a rigid body; they are entirely analogous to those of linear motion along a single fixed direction, which is not true for free rotation of a rigid body. The expressions for the kinetic energy of the object, and for the forces on the parts of the object, are also simpler for rotation around a fixed axis, than for general rotational motion. For these reasons, rotation around a...