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.
