As the spool - like rigid body is driven by the filament，its movement on the horizontal plane table ( or the horizontal rail) is a typical plane parallel motion. To change the inclination or way of the filament tension，the spool - like rigid body can present different motion states，which is an interesting demonstration experiment. The reason why the biaxial rigid body presents different motion states，the key factor is that the static friction dominates as the rigid body rolls on the desktop or the rail. The static friction can continuously vary in some ranges，so the linear movement of the rigid body can satisfy the kinetic equation and the kinematics equation. As the required staticfriction exceeds the maximum value，the spool - like rigid body will slide on the table. Also，the friction will change from the static friction to the sliding friction，and the pure rolling state will disappear.