The ratio relation 1:2 between rotation and wobbling frequencies of a torque free spinning plate or disc as seen from an inertial frame is calculated using a toy model for the disc. From a convenient rotating frame, the body oscillates as a pendulum in a centrifugal field, which simplifies the description of its movement. Thus, the ratio relation is simpler obtained when the system is first analyzed from a rotating frame and then from an inertial one. Using this approach, the derivation of the ratio relation for a flat disc is shown too. This appears useful when teaching elementary mechanics to undergraduate students because it avoids the difficulties that appear when the problem is solved using methods of analytical mechanics.
Keywords:
classical mechanics; spinning disc; rotating frame
