Physics
posted by Sarah .
a track is mounted on a large wheel that is free to turn with negligible friction about a vertical axis. 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 speed of wheel if its mass is M and its radius is R? solve using variables

If angular momentum is conserved, then the sum of the train anglular momentum must be equal and opposite to the wheel. The speed v must be the train speed v'and the tangential speed v" of the wheel.
Using v', and v", you can determine the angular speed of the wheel (remember you have R, m, and M). Assume the wheel to be either a hoop, or a disk. State which, the problem is not specific on this. I would go for the disk. 
thanks a bunch