Editing Screw theory (section)

=== Twist ===
A '''twist''' is a screw used to represent the velocity of a rigid body as an angular velocity around an axis and a linear velocity along this axis.  All points in the body have the same component of the velocity along the axis, however the greater the distance from the axis the greater the velocity in the plane perpendicular to this axis.  Thus, the helicoidal field formed by the velocity vectors in a moving rigid body flattens out the further the points are radially from the twist axis.

The points in a body undergoing a constant twist motion trace helices in the fixed frame.   If this screw motion has zero pitch then the trajectories trace circles, and the movement is a pure rotation.  If the screw motion has infinite pitch then the trajectories are all straight lines in the same direction.