  We first present a way to formulate the equations of motion for a
nonholonomic system with nonlinear constraints with respect to the velocities.
The formulation is based on the Cetaev condition which aims to extend the
practical method of virtual displacements from the holonomic case to the
nonlinear nonholonomic one. The condition may appear in a certain sense
artificial and motivated only to coherently generalize that concerning the
holonomic case. In the second part we show that for a specific category of
nonholonomic constraints (homogeneous functions with respect to the generalized
velocities) the Cetaev condition reveals the same physical meaning that emerges
in systems with holonomic constraints. In particular the aspect of the
mechanical energy associable to the system is analysed.
