Final answer:
The centripetal force on an unbanked curve is supplied by friction between the tires and the road.
Step-by-step explanation:
The centripetal force on an unbanked curve is supplied by friction between the tires and the road.
In this case, the friction between the tires and the road acts as the centripetal force, keeping the car moving in a circular path. The maximum static friction (at which the tires roll but do not slip) between the tires and the road provides the necessary centripetal force to keep the car on the curve.