18. In a lecture demonstration, a toy train track is mounted on a large wheel that is free to turn with negligible friction about a vertical axis; see Fig. 10-25.
A toy train of mass m is placed on the track and, with the system initially at rest, the electrical power is turned on.
The train reaches a steady speed v with respect to the track. What is the angular velocity  of the wheel, if its mass is M and its radius R?
(Neglect the mass of the spokes (wires) of the wheel.)