I have a conceptual issue with wheel friction that has been bothering me for a while. Consider the wheels on a car set to cruise control such that they rotate with constant angular velocity. Neither the wheel nor the ground are deformable (so we can ignore rolling friction) and the wheels slip while rolling. I understand that if the car is driving on a constant slope and the wheels are slipping that the friction between the wheels and surface is dynamic. But is this still true on a bumpy surface where the car has to constantly accelerate to maintain angular velocity? If the wheel is accelerating, then doesn't there have to be at least some static friction between the ground and the wheel? Any help would be greatly appreciated. Thanks! Edit: I guess what the question boils down to is: if a wheel is rotating and slipping but also being acted upon by an external force (a car accelerating), then is the friction static or dynamic?