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### Centripetal Force and Centrifugal Force

Centripetal force is the force present in circular motion. Centripetal force is always pointed inward, towards the center of rotation. A centripetal force causes a centripetal acceleration. Any acceleration causes a change in velocity. Since the centripetal force is always pointed inward, radially along the circular path, it is always pointed perpendicularly to the velocity vector (which is always tangent to the circle), which makes it possible to have a constantly changing velocity with a constant speed.

Centrifugal force is the reaction to the centripetal force. This is equal in magnitude to the centripetal force, directed away from the center of rotation, and is exerted by the rotating object upon the object which exerts the centripetal force. As it is an actual force, it is always present, independent of the choice of reference frame.

The fictitious/virtual centrifugal force appears when a rotating reference frame is used for analyzing the system. The centrifugal force is exerted on all objects, and directed away from the axis of rotation.

Both of the above can be observed in action on a passenger riding in a car. If the car swerves around a corner, the passenger's body pushes against the outer edge of the car. This is the reactive centrifugal force, which is called a reaction force because it results from passive interaction with the car which actively pushes against the body.