The acceleration of the car is unknown, so we can’t determine the linear force on the pebble. If we assume the translational acceleration is small compared to the apparent centrifugal force which constrains the pebble to a circular motion, this force is given by mv²/r, where v is the translational speed, r is the distance from the object to the perpendicular axis of rotation and m is the object’s mass.
So 3.6=0.0012v²/0.76. So v=√(3.6×0.76/0.0012)=47.75 m/s approx. (about 172km/hr).
This is the speed that generates a tangential force of 3.6N to balance frictional force.
Let’s see what linear acceleration would be required to produce a force of 3.6N.
a=3.6/0.0012=3000m/s² which is 3km/s every second, or 10,800kph every second! Hardly a gradual acceleration, so we seem to be justified in ignoring linear acceleration.
Gravity can be ignored for the same reason since it is less than 10m/s every second.