It wasn't for a school assignment (i'm not that lame). I'm building a sumo bot using Arduino and I'm trying to chose the best wheels. Assuming that the same motor was used would the smaller wheels have more torque to push the other robots.
Was just messin' ;-)
My understanding is that torque is measured as force x distance. foot-pound, oz-inch, kg-cm, Nm are examples of units you will often see.
The torque of the motor is a constant at a constant voltage / current, so as the distance increases, the force is reduced.
eg: 20 kg-cm at 1 cm affords 20kg of force. At 2 cm, you're going to max out at 10kg. Assuming constant current.
So the smaller wheels should afford you more force at the circumference of the wheel, but the torque is the same.