  A generalization of Coulomb-Amontons' law of dry friction recently proposed
by V. V. Kozlov is considered in the context of rigid body dynamics. Universal
requirements for dry friction tensor formulated by V. V. Kozlov are
complemented by a condition taking into account the contact nature of dry
friction and applied to several models. For the famous Painleve problem a
generalized Coulomb-Amontons' force without singularities, yet such that the
dissipation takes place only at the point of contact, is found. By the example
of the motion of a rigid ball on a plane with a single point of contact, it is
shown that these principles are consistent with the well-known equations,
studied by G.-G. Coriolis. Further, a ball simultaneously touching two
perpendicular planes at two points of contact is considered. The corresponding
equations of motion are derived and analyzed. An exact particular solution that
describes a technique used in practice in billiards is obtained. It is shown
that unlike the single contact case, the Lagrange multipliers can depend on
friction coefficients. With this in mind, a generalization of the conditions on
the tensor of dry friction for the case of an arbitrary number of contacts is
proposed.
