With this made clear, we can take a closer look at the so called "centrifugal forces", that arise when an object rotates around a center, to which it is mechanically connected, for example a flywheel. In fact, centrifugal forces cannot exist (they are reaction firces instead) - there is only a centripetal force that keeps, the object in its circular motion, changing the direction (state) in which its kinetic energy exists. In the following we will see that similar is valid for gravitation.
If firces (centrifugal) and forces (centripetal) would be identical in the physical sense, taking each other out, there would be no resulting force to keep the object in circular motion. We can now see that it is crucial to use the term "firce" to make the distinction and to eliminate confusion. In lack of this term's existence in scientific vocabulary, even a top-physicist recently opposed me by pointing out that in the contact area between a rotating object and its mechanical connection to the center of rotation, Newton's third law applies, making centrifugal "forces" to be a physical reality - well then, try to measure it as such!
From now on we can talk in terms of "centripetal forces" and "centrifugal firces", by which this widely spread confusion, ever since the days of Newton, finally can be eliminated (Summary & Conclusions - item 1).
When we consider the situation for objects in gravitational orbit, the term "firce" gets its final physical significance. It is a known fact by experience, that astronauts are weightless in their orbiting space crafts and in popular science it is claimed that the cause of this is, that the gravitational "force" is taken out by the centrifugal "force" working on the object (astronaut) in orbit. Evidently, there is no mechanical connection between the orbiting object and the center of orbit on one hand, and gravitational "forces" do not generate inertia on objects in free fall on the other hand. Hence, there is no gravitational "force" and in consequence there can be no balance with centrifugal "forces" either (even though it calculates correctly - mathematics are a tool IN, but not a proof OF physics).
Also from this point of view follows that centrifugal forces do not exist. Centrifugal firces on the other hand, do exist (as inertia) and then also gravitational firces can exist as weight. With this totally different approach, we get a beautiful consistency with Albert Einstein's equivalence principle. Even more so, as we now can see that there are no heavy and inert masses; there is mass only!
Hence, objects in gravitational orbit are not pulled into orbit by gravitational forces, just as little as they are "thrown out" by centrifugal firces. Firces have no potential to change the mechanical energy of the objects on which they are generated (mind .. not working). For the same reason a gravitational field can not be an energy source; what ever comes out, must be put in first and so "gravitational energy" is a meaningless expression. Objects in gravitational orbit are simply moving according to earlier mentioned definition(2), because their mechanical energy remains constant. (in General Relativity, they follow sort of a straight path in a curved space-time).
In consequence, from the concept of inertia arising from changes in mechanical energy only, from which the definition of force and firce follows, the equivalence between inertia and gravity follows also, as will be more clarified further on. Inversely, from the fundamental postulate in General Relativity that inertial and gravitational mass are equivalent and indistinguishable; the so-called principle of equivalence, follows that inertia must be caused by changes in mechanical energy. An according consistency with General Relativity is still missing in the present ZPF-theory of inertia.(2)
This consistency should not surprise us if we more in detail consider what actually happens in Einstein's accelerated cabin in free space, free from any gravitational influences. A test person in that cabin, who drops a ball, will surely observe it to fall to the floor of the cabin, but we can all agree that there are no forces working on that ball (the same as the skating observer in the braking vehicle) - it simply moves according to Newton's first law, or rather according to previous mentioned definition(2). The ball is not attracted by the floor of the cabin and by equivalence, neither is an object in free fall attracted by the center of a gravitational field. Once the floor of the cabin makes contact with the ball (not the other way around !) it will increase the mechanical energy, in this case the kinetic energy of the ball, causing inertia by which the ball gains weight (firce). By equivalence, the same thing must be valid for an object at rest in a gravitational field - we have weight because the surface of the Earth changes our mechanical energy, causing inertia (weight).
(What the nature of this change is, is still an open question to me - I am working on that in a new cosmological theory, involving three-dimensional time.)
Observe, that this inertia (weight) is a firce and as such detectable, whereas the causing force, which is the essence of gravity, cannot be detected in our world of observations - we need a new cosmological model (!). At this point we can find an answer as to why inertia occurs with mechanically rotating objects. The centrifugal firce is definitely an inertial one, although the amount of kinetic energy of the rotating object is constant. If we however, resolve the motion of the rotating object on the axes of a preferred reference system of observation, we see that the resolved components of the motion, constantly change between zero and a maximum along the according axes. Because there is no potential energy component, that compensates for the according changes in kinetic energy - unlike objects in gravitational orbit - inertia arises (Summary and Conclusions - item 3). Let's illustrate this on the next page, by considering ballistic motions.
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