[Bar90b]
Cornell University Program of Computer Graphics 

Determining frictional inconsistency for rigid bodies is npcomplete.David Baraff.Technical report TR901112, Department of Computer Science, Cornell University, April 1990. The computational complexity of computing the forces between bodies in contact is presented. The bodies are restricted to be perfectly rigid bodies that contact at finitely many points. It has been known for some time that under the Coulomb model of friction, some configurations of bodies are inconsistent; that is, no contact forces satisfying the constraints of the Coulomb friction model exist for the configuration. The main result of this paper is a proof that determining if a configuration is inconsistent is an NPcomplete problem. An immediate corollary of this proof is that computing the contact forces for a configuration of bodies is NPhard. Computing contact forces remains NPhard even if configurations are restricted to be consistent. The Thesis is available online from the Cornell University Department of Computer Science as Technical Report TR901112.  
