Relative velocities and accelerations define the relationship of two objects, not which one is actually moving or if both are. Acceleration of an object is measured from the object's frame of reference.
The only measure of an object falling as accelerating, follows from establishing a frame of reference other than that of the object, as a rest frame and then comparing the falling object to that rest frame.
An accelerometer measures a falling object's acceleration as zero, until it hits the ground. At that time the object experiences an extreme acceleration.., and usually significant damage.., before it once again comes to rest in its own frame of reference.
Could you try and explain that again for I have always understood thing accelerate as they fall @9.8 m/sec^2.
So if the accelerometer says when something is falling it is not accelerating, was it accelerating when it was stationary before it commenced falling? Sounds like an object can go from 0 to 100 Km/h without accelerating??
I am really struggling with this one. Can someone give me a logical answer please?
I must apologize, it often occurs that when I post I am focused on a specific issue and fail to recognized or at least acknowledge the more general situation. Acceleration can be one of those issues that is valid from conflicting frames of reference, without that being entirely a relativity issue. Part of the difficulty is that the terms and ideas involved when discussing mass, inertia, acceleration and gravitation, are somewhat circular and interdependent.
The simple answer, if anything here is simple. Accelerometers measure acceleration as a function of an object's resistance to a change or changes in velocity. When an object is in the free fall of a stable orbit or falling toward a gravitational center of mass, in vacuum, it experiences no inertial resistance to the acceleration and thus an accelerometer, cannot measure the rate of acceleration.
The object is accelerating and it is still subject to inertial resistance to that acceleration, however an accelerometer cannot detect the acceleration.
Emil was right when he questioned and corrected my statement,
An accelerometer measures resistance to a change in motion.., which is what acceleration is.
The portion struck out should have been left out.....
Back to the question,
Accelerometers do not measure acceleration in free fall, because the gravitational interaction responsible for the acceleration, acts on every part and atom of the object equally. Gravity results in an acceleration toward the center of gravitational mass, what we feel both when standing on the surface of the Earth or within a spaceship that is accelerating; In the case of gravitation is the ground stopping us from accelerating toward the Earth's gravitational center of mass. Or in the case of the spaceship, the floor of the ship pushing us in the direction of the space ship's acceleration.
It begins to get somewhat complex when comparing these two situations at a level more fundamental than "experience". While in the spaceship we are accelerating and feeling a resistance to that acceleration as the floor pushing up, when standing on the Earth, if we set aside the Earth's rotation and astronomical velocities, we are actually at rest, and feeling the inertia of the gravitational interaction with the Earth, as the ground preventing any acceleration toward the Earth's gravitational center of mass. So in one case we can say we are accelerating and the other we are not.., and yet both result in the same experience, of the ground or floor pushing up against our feet.
Accelerometers as we have been discussing can only detect acceleration when it is not the result of a gravitational interaction, that is not being resisted. Accelerometers essentially measure mass as weight, derived from resisting the acceleration.
