by Matthew Vannette

Created on: January 21, 2007   Last Updated: May 02, 2007

The "centrifugal" force experienced by an object moving along a curved path is only present in the rotating reference frame of the object. The fact that it does not exist unless you are in the rotating frame is evident from an analysis of Newton's laws.

Newton's First Law (N1L) - An object at rest tends to stay at rest and an object undergoing uniform linear motion will continue in that motion along a straight line unless acted upon by an outside force provided one is in an inertial reference frame.

This is important for the understanding of rotation because it says that something moves in a straight line at constant velocity (both how fast and the direction of motion) unless some other thing exerts a force on it. That is, you need two different bodies to change the state of motion. An inertial reference frame is best defined as a reference frame where Newton's First Law is valid. That is to say, things do not change velocity unless there is an obvious external agent. Strictly speaking, Earth is not an inertial reference frame because things fall to the floor unless some force holds them up. It is obvious to us that gravity pulls the object down, but this is only true because we live in a post-Newton time.

Newton's Second Law (N2L) - The total acceleration an object experiences is equal to the force exerted on that object divided by the mass of the object. Further the acceleration and the force point in the same direction.

What does this mean for the current discussion? First, in order for an object to accelerate there must be a non-zero force on it. Also, the direction of the acceleration is the direction of the force. So, by looking at the acceleration of an object we can see which direction the net force points.

Newton's Third Law (N3L)- If one object (A) exerts a force on a second object (B) then B exerts a force equal in magnitude and opposite in direction on A.

The third law is the least understood law. At first glance it would seem to suggest that there is no net force because forces show up in equal and opposite pairs. Well, if there are two forces equal in strength but pointing in opposite directions then the net force should be zero, and there should be no acceleration (N2L), right? Not quite. The third law works on two different bodies while the second law is concerned with the forces on a single body.

So, what does this have to do with anything? First, Newton's laws are only valid in an inertial reference frame or in a frame where you can explicitly consider

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