An apparatus for developing inertial forces, the resultant of which is a unidirectional force, includes at least two masses (M) or pairs thereof, each rotating on a generic plane along a closed path, the path included between minimum and maximum paths, both included in an outer circumference (C1) having a radius (R1) and a diameter (D1) perpendicular to the symmetry axis of the path of (M), as well as a center (O) coinciding with the rotational center of (M). The minimum path is defined by a circumference (C3) having a radius (R3) equal to 1/10 of (R1), and a center (O′), set on the axis of symmetry of the path of the masses and tangential to the center of (C1). The maximum path is defined by two minor circumferences of radius (RK)≦¾ (R1), each having its own center on the diameter (D1) of (C1) and being tangential thereto.