1. Originally Posted by origin
Hmm, could you point to where the furthermost extremities of the universe are? Hint: you can't, anymore than you can point to the center of the universe.
You can go there in your imagination. Imagine what it is like. What do you see? All the galaxies are now on one side of you, looking the other way all you see blackness.

Ok it might be a little trickier to point you in the right direction, but I think it is possible to work it out.
The universe is rotating on an axis, so if it is like other structures within itself, it is rotating. It probably has a distinct oblateness even to point of being disk like.

2. Originally Posted by Robittybob1
It gets a bit more difficult to understand how an object in free fall is accelerating but an accelerometer will register nil acceleration during free fall.
I can only think that is due to the design of the accelerometer.
In my opinion, you are right.

What is acceleration?
Definition: "When the velocity of an object changes it is said to be accelerating. Acceleration is the rate of change of velocity with time."

"Acceleration is the derivative of velocity with time, but velocity is itself the derivative of displacement with time. The derivative is a mathematical operation that can be applied multiple times to a pair of changing quantities. Doing it once gives you a first derivative. Doing it twice (the derivative of a derivative) gives you a second derivative. That makes acceleration the first derivative of velocity with time and the second derivative of displacement with time."

3. Originally Posted by Emil
In my opinion, you are right.

What is acceleration?
Definition: "When the velocity of an object changes it is said to be accelerating. Acceleration is the rate of change of velocity with time."

"Acceleration is the derivative of velocity with time, but velocity is itself the derivative of displacement with time. The derivative is a mathematical operation that can be applied multiple times to a pair of changing quantities. Doing it once gives you a first derivative. Doing it twice (the derivative of a derivative) gives you a second derivative. That makes acceleration the first derivative of velocity with time and the second derivative of displacement with time."
We need to understand what the real definition of Free Fall is too?

http://en.wikipedia.org/wiki/Free_fall is a start.

4. Originally Posted by Robittybob1
We need to understand what the real definition of Free Fall is too?

http://en.wikipedia.org/wiki/Free_fall is a start.
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.

5. You can go there in your imagination. Imagine what it is like. What do you see? All the galaxies are now on one side of you, looking the other way all you see blackness.
You don't learn anything, do you?

6. Originally Posted by Robittybob1
We need to understand what the real definition of Free Fall is too?

http://en.wikipedia.org/wiki/Free_fall is a start.
Yes,
It is the direct action of the universal attraction force. (Without something to be interposed between them.)
This explains the transformation of potential energy into kinetic energy.
In free fall, the speed of a body increases due to gravitational acceleration and potential energy is converted into kinetic energy.

7. Originally Posted by OnlyMe
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.
Yes, this is valid for any object that is accelerating.
Originally Posted by OnlyMe
An accelerometer measures...
How do you think it works this accelerometer?
How measures "the rate of change of velocity with time" ?

8. Originally Posted by AlexG
You don't learn anything, do you?
How can it be anything else?

9. Originally Posted by OnlyMe
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?

10. Gravity can be 0 (zero)!
Can any physicist with degree, explain that?

11. Originally Posted by Emil
Yes, this is valid for any object that is accelerating.

How do you think it works this accelerometer?
How measures "the rate of change of velocity with time" ?
An accelerometer measures resistance to a change in motion.., which is what acceleration is.

There is in a sense another expression of acceleration which cannot be detected by an accelerometer. This is the free fall experienced by a stable orbit, where the gravitational interaction is counterbalanced by an opposing centrifugal force. The centrifugal force ofcorse being a ficticiuos force derived by an angular acceleration, is experienced as inertial resistance to the constant angular acceleration. However, in that particular situation the gravitational interaction and centrifugal force oppose one another, cancelling out the experience normally associated with either angular acceleration or remaining stationary in a gravitational field by any means.

12. Originally Posted by OnlyMe
An accelerometer measures resistance to a change in motion.., which is what acceleration is.

There is in a sense another expression of acceleration which cannot be detected by an accelerometer. This is the free fall experienced by a stable orbit, where the gravitational interaction is counterbalanced by an opposing centrifugal force. The centrifugal force ofcorse being a ficticiuos force derived by an angular acceleration, is experienced as inertial resistance to the constant angular acceleration. However, in that particular situation the gravitational interaction and centrifugal force oppose one another, cancelling out the experience normally associated with either angular acceleration or remaining stationary in a gravitational field by any means.
I understand that to be weightlessness. A person on board a craft in the balanced state will experience weightlessness.

13. Originally Posted by Emil
Gravity can be 0 (zero)!
Can any physicist with degree, explain that?
Gravity is never zero.

14. Originally Posted by OnlyMe
An accelerometer measures resistance to a change in motion.., which is what acceleration is.
As I posted earlier, the definition of speed and acceleration is: "From the instantaneous position r = r (t) (instantaneous meaning at an instant value of time t), the instantaneous velocity v = v (t) and acceleration a = a (t) have the general, coordinate-independent definitions: "
As noted in this definition does not exist and no need for mass.

Resistance to a change in motion is inertia.
There is a correlation between mass and inertia.

From a practical perspective, it is difficult and expensive to make a device to measure acceleration as defined, considering the conditions exposed also to you, in post no.444

Accelerometer, as you described, measure the inertia of a mass standard.
Force that causes acceleration is not allowed to act on the mass standard. For instance, standard weight is suspended between springs.

Once acceleration, standard mass exerts a force on the springs.
This force is "read" and divided by the value of standard mass. In this way we get the value of acceleration. Considering only this device, we will not know if acceleration or deceleration.

But if we cannot isolate the force that produces acceleration, not to act on the mass standard? (This happens for gravity.) In this case we can not use this device.

15. Originally Posted by AlexG
Gravity is never zero.
In a frame free falling gravity is zero. For the occupants will experience weightlessness and can't tell if they are moving or accelerating and at the same time they will have inertial mass but no gravitational mass. So in all respects they are in a zero gravity situation.

16. Originally Posted by Robittybob1
In a frame free falling gravity is zero. For the occupants will experience weightlessness and can't tell if they are moving or accelerating and at the same time they will have inertial mass but no gravitational mass. So in all respects they are in a zero gravity situation.

17. Originally Posted by Robittybob1
In a frame free falling gravity is zero. For the occupants will experience weightlessness and can't tell if they are moving or accelerating and at the same time they will have inertial mass but no gravitational mass. So in all respects they are in a zero gravity situation.
Alex I think you will find I'm right.

free fall ..>>>>> no acceleration detected by accelerometer
free fall >>>>>> weightlessness
Relativity (according to Confused2) you can't tell if it is you who is moving
All physics will be normal in your frame >>>> inertial mass can be calculated.

So how would you be certain there was gravity in that situation?

18. Robitty, free fall means there is nothing resisting gravity. You feel weightless not because there's no gravity, but because you are freely falling under the influence of gravity.

19. A clue for gravity zero.
If we consider Earth as a closed system, then in the center of the Earth (we consider homogeneous Earth), the gravity is zero.

20. Originally Posted by Robittybob1
Originally Posted by OnlyMe
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,
Originally Posted by OnlyMe
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.