The flywheel (3) comprises a flywheel mass (5) and an automatic starting device (9) for driving into rotation the flywheel mass (5), coaxial with the flywheel mass (5), rotatable with respect to the flywheel mass, and adapted to be torsionally coupled to a motion input (7). The automatic starting device (9) comprises a drag torque transmission member for transmitting drag torque from the automatic starting device (9) to the flywheel mass (5), adapted to transmit a torque that increases as the angular speed of the automatic starting device increases.