Abstract: The problem of a bug crawling on a plane motion rigid body is studied by the law of conservation of momentum and the law of conservation of angular momentum. By which, the differential relation between the rotation angle of the rigid body and angular coordinates of the bug in the rotating coordinate system is derived. Moreover, the formula of the rotation angle is obtained as the trajectory of the bug on the rigid body is straight, and the application of the formula is presented by given typical example.

Key words: law of conservation of angular momentum, rotating coordinate system, angular coordinate