A spaceship is moving horizontally with uniform acceleration. Inside the spaceship, a body m is pushing against the rear wall. This force is action, and is inertial force. As reaction, and as normal force, the rear wall pushes back against the body. Magnitude of the two is ma.
Above explanation should be valid for one inside and outside the spaceship. Inertial force will be a real force for everyone, and will not be fictitious.
A body is being pulled by strings from the left and right. Tension in the strings is the same, ma. The same body is being pulled by one string from the left and by two strings from the right. Tension in all three strings is the same, ma. Let the body in two figures be A and B.
Are A seen from B and B seen from A symmetrical ? No, that's not, because the internal stresses in body A and B are different.
A spherical fluid is floating in space. The fluid is uniform and isotropic. That is, gravity has no acting on the sphere.
The sphere will be in non-accelerated frame. It is moving in a uniform linear motion through space (or is at rest). However, if the sphere moves in a way other than the above, there will be a corresponding deviation from uniform isotropy. The difference between accelerated and non-accelerated frame is not fictitious.
Spaceship is moving horizontally with uniform acceleration. Body m is pushing against rear wall of the spaceship's cabin (A). Rear wall pushes back the body with normal force (B). A is action, B is reaction.
Spaceship is moving horizontally with uniform acceleration. String attached to front wall of the spaceship's cabin stretches backward inside the cabin, pulling body m attached to rear end of string (A). The body resists pulling force of string (inertial resistance) (B). A is action, B is reaction.
To begin with, distinction between action and reaction seems to be unclear.
In weightless space, there is a starting point. From this point to six directions (east-west, north-south, and up-down directions) six points of mass start motion simultaneously at the same speed. The state of six points of mass will show the state of non-accelerated frame. These will also be an indicator for all accelerated motion (for all not uniform linear motion).
If desired, the starting point can be made stationary on aether frame.
On a plane, two rods cross each other (without friction) and rotate around intersection point. This basic form can be transformed as follows.
1) Body of mass m is attached to both ends of one rod. Body of mass 2m is attached to both ends of the other rod. Mass of rods is assumed to be zero.
2) Body of mass m is attached to each of four ends of two rods. Length of one rod is a, and length of the other is 2a. Mass of rods is assumed to be zero.
Centrifugal force and tension acting on rods are real forces, not fictitious. From any frame.
Centrifugal force that is accompanied by rotational motion is inertial force, that is, real force or action. Reaction to it is a centripetal force (In case of hammer throwing wire, it can also be considered tension or tensile stress)
. Its form and magnitude do not vary no matter what frame it is viewed from.
Following claims : Newton’s spherical shell theorem, Perihelion shift of Mercury (claimed before relativity, before 1905) are unacceptable. My veiw are posted into some web-sites in English.
Also in relativity, I find no acceptable claim regarding gravity (equivalence principle is unacceptable). And I posted objections (that I can) to relativity’s claims regarding gravity.
"Resultant force of attraction and centrifugal force of Earth is the true nature of gravity." This is cited from a website. However, I think that attraction in this citation is the true nature of gravity. This citation is evasive and delegate.
Equivalence principle is unacceptable. One reason is that, I can't recall ever seeing universal gravitational constant mentioned on inertial force.
On state of being of vectors of gravity (attraction) and inertial force, there will be no restrictions. Therefore, vector of two forces will be possible to be compensate and nothing special happens. Like an elevator in free fall.
Force acting on us on surface of Earth is resultant force of gravity (attraction) and centrifugal force. On surface (solid or liquid) of planets of solar system, gravity (attraction), and centrifugal force (different planet’s rotation) is various. Equivalence principle is unacceptable.
An elevator cabin is in free falling. Elevator is rigid body consisting of the same mass points with mass m. Each mass point has the same acceleration. The same force acting on each mass point is ma.
There is no word equivalence principle in above explanation or any of the following. In fact, there should be no word equivalence principle.
A body of mass m is placed on flat surface (without friction). String pulls this body to the right with tension ma. The body accelerates to the right, and inertial force ma acts to the left. Magnitude of tension and inertial force are equal. This is as shown by Newton's third law of motion.
The fact that gravity and inertial force are equal (in total) in a free-falling elevator is probably due to Newton's third law of motion.
The two, gravity and inertial force can be expressed as force vectors. So, it is possible that the two cancel each other out in a free-falling elevator (as a phenomenon).
However, free falling is only one phase in which two vectors interact. There is no reason why free falling should be treated as something special.
Kepler's third law states that orbital speed of a planet depends on radius of its orbit (if orbit is perfect circle). However, formula for second cosmic velocity is depends on radius of Earth. Are these two compatible ?
On a celestial body below, two elevator cabins the same are falling freely. They are lined up one above the other and connected by a long string. Is some point on the string local inertial frame ?
Pearl necklace is falling freely in a vertical line. Gravity acting on each pearl is mM/r^2, and inertial force is ma. Mass of the string holding pearls is zero.
This post does not refer to equivalence principle.
Formula for gravity, G mM/r^2, and formula for inertial force, ma, are acting on pearl necklace. Gravity acts on each pearl with the magnitude indicated by the formula.
It is inevitable that gravity and inertial force cancel each other out totally (apparent) at a specific mass point.
For free falling, pearl necklace would be a better thought experiment than elevator. There will be other examples too. A real example is Comet Shoemaker-Levy 9, which fell on Jupiter in 1994.
Common theory of Mercury's perihelion shift is poor. Also, Newton's shell theorem should be reconsidered.
We can say that we can quantitatively grasp both gravity and inertial force. However, we cannot qualitatively grasp them. Do we have the right to preach equivalence principle ?
In fact, perhaps equivalence principle can be directly denied.
