http://forums.studentdoctor.net/showthread.php?p=5304821
Xanthines: Here is an explanation of centripetal force, using the example of your car making donuts in a level parking lot. In this case, the engine is providing the power for forward motion (by forward, I mean the direction of motion of the front of the car). The centripetal force equation Fc=(m)(v^2)/r describes the force required to keep the object in a circular path. The static friction force points towards the center of the circle, and this is the source of the centripetal force that keeps the car moving in its circular path for this example:
Fc = uFn = mv^2/r
Without a centripetal force, objects such as cars and yo-yos on strings will move in a direction that is tangential to the circular path they were on previously. Basically, what happens is that when you let go of the steering wheel, the car will move in a straight line that is tangential to the donuts you were making before. The car is not actually being pushed away from the center of the circle; rather, the centripetal force had been holding it in its circular motion, and now that centripetal force is gone. In addition, if the car moves fast enough, the force due to static friction will not be high enough to maintain circular motion. Again, that is because on level surfaces, the centripetal force is the friction force...I've heard the term "centripetal force requirement" used to describe this situation. This example shows us why it is that without friction, there can be no circular motion for cars. For example, when you try to make turns on icy patches of the road, you find that you cannot turn in a circle. There is no friction to provide a centripetal force, and thus you skid off the road into a ditch. Note that this is only true for level surfaces. On banked highways and race tracks, gravity comes into play.
To sum: centripetal force isn't really a new type of force, but rather is just the name used to indicate the net force pointing inwards for an object in circular motion. In the case of a car making donuts, the centripetal force is the static friction force. Since the static coefficient of friction is normally given as the maximum, you know that any more force applied to that object will overcome the friction, and the object will begin to slide in a direction tangent to the circle of motion (or in the case of the car, skid off the road.) Analogously, when a satellite moves fast enough, it will escape the earth's gravity:
G(Mearth)(Msatellite)/(r^2) is less than (msatellite)(v^2)/r
One final point: When most people use the term "centrifugal force," they are actually referring to what is commonly referred to by another physics term known as inertia.
Shrike: It's true that by "centrifugal force," non-physicists usually mean something else: either inertia, or centripetal force. But to clarify what Xanthines said, there *is* such a thing as centrifugal force. Recall that, by Newton's Third Law, for every force there's an equal and opposite force, and that the two constitute an action-reaction pair. Centrifugal force is the force that forms a pair with the centripetal force -- it's the force exerted by the body that's moving in a circle, on whatever's making it turn.
For example, in the case of a yo-yo whirled on a string, the force of the string on the yoyo is centripetal, and the force of the yo-yo on the string, and thus your finger, is centrifugal. In the case of the car driving in a circle, the ground exerts a centripetal foce on the tires, while the tires exert a centrifugal force on the ground. You might guess from these examples that it's centripetal force that we usually worry about, not centrifugal. You'd be right. Centrifugal force tends to be a little odd, and mostly irrelevant. I've never seen it matter on an MCAT problem.
