Another force, which appears to be opposite the direction of motion, is the centrifugal force acting on an object that follows a curved path. This force appears to be a force directed away from the center of the circular path. This is actually a fictitious force, but is an apparent force that is used to describe the forces present due to an object's rotation.

Figure 11 Centrifugal Force

To better understand centripetal and centrifugal forces, consider that a string is attached to the plane in Figure 10. As the plane rotates about the center, the string places a centripetal force on the plane. This causes the plane's velocity to change in direction, thus causing it to travel in a circle.

The apparent outward force, centrifugal force, seems to pull the plane away from the center shown in Figure 11. This is the same apparent outward force one feels when riding in a car when the car travels in a circle. It can be proven that centrifugal force is not an actual force by cutting the string. In doing so, the plane will fly off in a straight line that is tangent to the circle at the velocity it had the moment the string was cut. If there were an actual centrifugal force present, the plane would not fly away in a line tangent to the circle, but would fly directly away from the circle (see Figure 12).

Figure 12 Loss of Centripetal Force