Physics Tutorial>>Is Centrifugal Force Real?

Centrifugal force has been maligned by scientists, teachers, and lay people alike. Arguments denying the existence of centrifugal force are either groundless or are in conflict with tenets of classical physics. Lets give this concept clarification and the respect it deserves.

Here is one way that centrifugal force is maligned. Let us imagine a book on a slippery dashboard in a moving car. To a passenger in the car the book appears stationary until the driver makes a sharp turn to the left. At this point the book appears to the passenger to accelerate to the right. The passenger may be inclined to conclude that some force is accelerating the book away from the center of the car's curved path. Further the passenger may label the cause of this motion "centrifugal force."

At this point a well meaning teacher will explain that actually the book is not accelerating. It is going in a straight line, and the car is moving out from underneath it. The teacher may even point to this as an example of Newton's First Law of Motion: since there is no force acting on the book it moves in a straight line (or vice versa). So far so good, but now some trouble starts. The teacher may then say that since it only appears that the book is experiencing a force then centrifugal force is fictitious. This is over generalizing. The truth is there was no centrifugal force acting on the book any more than there was centripetal force acting on the book. This does not mean centrifugal force does not exist. This would be like saying that magnetic force is fictitious because there is no magnetic force acting on the book.

The same teacher will then go on to teach students how to draw free body diagrams, employing normal force to balance the weight of a stationary book on a horizontal table. Centrifugal force is just as real as normal force: both are reactionary forces. To deny the existence of either would be to deny Newton's Third Law of Motion. The table pushing up on the book is a reactionary force to the book pushing on the table. When a ball is whirled in circular motion at the end of a string, the string pulls on the ball and the ball pulls on the string. If the string pulling the ball inward is called centripetal force, what do we call the ball pulling outward on the string: "Fictitious."? I think not. The appropriate term for this center fleeing force would be "centrifugal force."

One popular text book in high school physics launches a groundless attack against this noble force complete with a diagram of a ball traveling in a circle at the end of a string. The authors claim that if centrifugal force existed, then when the string breaks the ball would fly away from the center of the circle. The authors even invite readers to do this experiment. Isn't it odd that the authors don't invite readers to lift a book off a table? Shouldn't normal force then make the book fly upward? Obviously there is some misunderstanding of the nature of reactionary forces. Just as normal force only exists and must exist when the book is pushing on the table, centrifugal force only exists and must exist when centripetal force exists. As soon as the string on the ball is broken the action and reaction forces both cease to exist and the ball continues in a tangent according to Newton's First Law.

Another source of misunderstanding about centrifugal force is that is does not appear in free body diagrams of objects undergoing circular motion. The argument goes: if it does not appear in free body diagrams, then it must not exist. Centrifugal force does not appear in free body diagrams of objects moving in a circle because it does not act on the object undergoing circular motion. It acts on the central object or on the connecting string.

Consider a ball attached to a spring and being whirled around in a circular path. Centripetal force is applied to the ball causing it to move inwards. Centrifugal force is applied to the spring making it stretch outwards. Centrifugal force is real. Whereas normal force is the reactionary force of a surface perpendicular to the surface; centrifugal force is a reactionary force of an object moving along a curved path, directed away from the center of curvature.

Thanks go to Gregory Shepertyky for inspiring these thoughts.