Linked from
The 27 pages that link to Rigid body, each with the reason it gives.
Axis of rotationNarrower topic: A fixed axis gives a simple description of a rigid body's rotation.
Euler anglesNarrower topic: Euler angles describe the orientation of such an object's body frame.
FulcrumNarrower topic: Lever analysis treats the bar as rigid while it rotates about the fulcrum.
Four-bar linkageNarrower topic: The standard linkage model treats each bar as a rigid body.