For frictional unilateral constraints, the normal contact forces are modelled by one of the methods above, while the friction forces are commonly described by means of Coulomb's friction law. Coulomb's friction law can be expressed as follows: when the tangential velocity U T is not equal to zero, namely when the two bodies are sliding, the friction force λ λ T is proportional to the normal contact force λ λ ; when instead the tangential velocity U T is equal to zero, namely when the two bodies are relatively steady, the friction force λ λ T is no more than the maximum of the static friction force. This relationship can be summarised using the maximum dissipation principle, as