I said I was going to drop out of this argument, but I can't help myself.
It seems to me that we should get some agreement on how this works for forces accelerating a body in a straight line before we worry about circular motion or gravity.
The Wikipedia article on reactive centrifugal force (linked in post 3 of this thread) says:
"In accordance with Newton's first law of motion, an object moves in a straight line in the absence of any external forces acting on the object. A curved path may however ensue when a physical [force] acts on it; this force is often called a centripetal force, as it is directed toward the center of curvature of the path. Then in accordance with Newton's third law of motion, there will also be an equal and opposite force exerted by the object on some other object,[1][2] such as a constraint that forces the path to be curved, and this reaction force, the subject of this article, is sometimes called a reactive centrifugal force, as it is directed in the opposite direction of the centripetal force.
Unlike the inertial force or fictitious force known as centrifugal force, which always exists in addition to the reactive force in the rotating frame of reference,
the reactive force is a real Newtonian force that is observed in any reference frame. The two forces will only have the same magnitude in the special cases where circular motion arises and where the axis of rotation is the origin of the rotating frame of reference. It is the reactive force that is the subject of this article."
Addition in [] and bolding are my edits.
The same applies to acceleration in a straight line. For any accelerating body there is a real nett unbalanced external force, and an equal and opposite real internal inertial reaction force. These forces exist and are measurable from any frame of reference, because frames of reference only affect apparent velocity and acceleration, they don't affect the real forces.
For any non-inertial frame of reference there is also an imaginary external force, which is imagined in order to make the laws of motion work. For a frame of reference accelerating at the same rate as the body, the imaginary external force has the same magnitude and direction as the real inertial reaction force, but they are not the same thing. One is a real force and can be felt and measured and the other is an imaginary force that has no physical effect on anything.
In my opinion the quoted Wikipedia article states this all reasonably clearly and consistently, other than referring to the fictitious force as an "inertial" force. Unfortunately the other Wikipedia articles on the subject are inconsistent, and written as though the imaginary force and the inertial reactive force were the same thing, which they are not.
It seems to me that this inconsistency in terminology is the source of widespread confusion on this issue.
Doug Jenkins
Interactive Design Services
