If a moving object is not exposed to any force, it moves with a constant speed and along a straight line. To move an object -for example a car- along a curve a centripetal force is needed to pull it out of the straight path. This force is brought up by the frictional force of the tires on the streets surface. As physical forces always come up in pairs, the counter part of centripetal force is the centrifugal force. It is as strong as the centripedal force. It is used to describe why, a passenger feels like being pulled to the outer side of a car going around a curve.