[Jay Lundy]

I'm only taking my first year of physics this year and I don't know much about motors but I'll give it a try.

First of all, I don't think the friction between the wheel and the ground has much to do with it. However, it becomes important when talking about traction, and at what point the wheels will start slipping. Friction is the only thing that allows for the transfer between rotational and translational movement. For example, if you ran your robot on a frictionless surface, the wheels would just spin and your robot would not move. Conversely, if you gave your robot a push on a frictionless surface, the wheels would not spin, but the robot would move.

Relating this to last year's game, the bridge was at an angle so N was less, so the force of friction was less, so the wheels slipped. Basically traction depends on the coeffecient of static friction between your wheels and the floor, your weight, and the angle of the bridge. If the force causing your wheels to rotate overcomes the coeffecient of static friction * N, then the wheels slip.

As for why the robot does not keep accelerating, I seem to remember seeing a chart that graphed torque vs. speed. As the speed got faster, the torque got lower. Eventually, there is no more torque and since T=Fd, there is no more force to cause the robot to accelerate.

Think of it like you trying to rotate something like a bike wheel. You can make it accelerate, but eventually the speed maxes out and you can't apply any force to make it go faster. I think the motor works similarly.

Basically, the only significant forces working against the robot is the friction in the wheel axle, and any other objects the robot may run in to. Although the coeffecient of friction in the ball bearing may seem small, it makes a difference when it has 130 lbs on it. Air resistance and the friction between the wheel and the floor are both very small and don't affect the robot much.

Of course, I may be wrong about any of this.