Slip Ratio and Tration Force

Greetings,

Searching for a slip ratio equation and its relation to the traction
force, I found two seemingly different formulas:

slip_ratio = (angular_velocity * object_radius -
longtitudinal_velocity) / longtitudinal_velocity
F = slip_ratio * traction_coefficient

And,

slip_ratio = (angular_velocity * object_radius -
longtitudinal_velocity) / (angular_velocity * object_radius)
F = engine_torque * wheel_radius * slip_ratio

First question that comes to mind is which one is right? Second, I've
used the first equation, however I didn't know what would the slip
ratio be if the velocity is 0.0 (Starting up). If it would be zero then
the car would never start in the first place. Also what value does the
traction coefficient usually have?
Now, if the second formula was the right one then the wheel would never
stop spinning, because when there's no engine torque it would mean that
the traction force would be zero, so the acceleration would never go
negative (When going forward). So how can I solve this problem?

As a side question, is there any other torque (that I need to worry
about) exerted on the wheel besides traction, engine and brakes?

Thanks,
Abdo Haji-Ali
Programmer
In|Framez