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Rotation around a fixed axis
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<p>Rotation around a fixed axis or axial rotation is a special case of <a href="/facts/Rotational_motion/yChkz49x">rotational motion</a> around an <i><a href="/facts/Axis_of_rotation/PGcg3IY5">axis of rotation</a></i> fixed, stationary, or static in <a href="/facts/Three-dimensional_space/KAqKWUbS">three-dimensional space</a>. This type of motion excludes the possibility of the instantaneous axis of rotation changing its <a href="/facts/Orientation_(geometry)/Tj24mTJ8">orientation</a> and cannot describe such phenomena as <a href="/facts/Nutation/YDogrCoD">wobbling</a> or <a href="/facts/Precession/44Sk1PMD">precession</a>. According to <a href="/facts/Euler%2527s_rotation_theorem/t0moTljA">Euler's rotation theorem</a>, simultaneous rotation along a number of stationary axes at the same time is impossible; if two rotations are forced at the same time, a new axis of rotation will result. </p><p>This concept assumes that the rotation is also stable, such that no <a href="/facts/Torque/KcrcvNtV">torque</a> is required to keep it going. The <a href="/facts/Kinematics/o7mSyeJA">kinematics</a> and <a href="/facts/Dynamics_(mechanics)/JLk81H71">dynamics</a> of rotation around a fixed axis of a rigid body are mathematically much simpler than those for <a href="/facts/Rigid_body_dynamics/ajbkweF2">free rotation of a rigid body</a>; they are entirely analogous to those of <a href="/facts/Linear_motion/Vom0QwWe">linear motion</a> along a single fixed direction, which is not true for <i>free rotation of a rigid body</i>. The expressions for the <a href="/facts/Kinetic_energy/aJRxUd4p">kinetic energy</a> 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 <a href="/facts/Linear_motion/Vom0QwWe">linear motion</a>; the full generality of rotational motion is not usually taught in introductory physics classes. </p>