Clarification on Gravity

Quote from: sokarul on September 22, 2008, 10:17:21 AM

They would each see the other as accelerating.  But the free-faller is the one who is really accelerating based on a coordinate system.  That is why freefallers at both poles of the earth will hit the ground.

Did you read the EP?

Did you read the EP?

I am familiar with the Equivalence Principle, thanks. You are the one who doesn't seem to understand its implications.

They would each see the other as accelerating.  But the free-faller is the one who is really accelerating based on a coordinate system.  That is why freefallers at both poles of the earth will hit the ground.

What co-ordinate system? Whose rulers and clocks are we using? In any system, only one of them is accelerating in any meaningful sense, and that is the guy on the ground.

As for your second point - what are you talking about? Where did the 'both poles of the Earth' thing come from?

Have you ever used one of those simple force scales? 

Yes, they come factory-calibrated to read zero at sea level.  See wiki:

As a consequence, the output of an accelerometer has an offset due to local gravity. This means that, perhaps counter-intuitively, an accelerometer at rest on the earth's surface will actually indicate 1 g along the vertical axis. To obtain the acceleration due to motion alone, this offset must be subtracted.

Quote from: sokarul on September 22, 2008, 10:21:25 AM

Did you read the EP?

I am familiar with the Equivalence Principle, thanks. You are the one who doesn't seem to understand its implications.

Why then can you not comprehend that free-fall has acceleration?  Do you think the word "fall" is just in there to look good? 

Why then can you not comprehend that free-fall has acceleration?  Do you think the word "fall" is just in there to look good? 

Falling does not mean acceleration. Indeed, it means the opposite.

A person in a free-falling elevator experiences weightlessness during their fall, and objects either float alongside them or drift at constant speed. Since everything in the elevator is falling together, no gravitational effect can be observed. In this way, the experiences of an observer in free fall are indistinguishable from those of an observer in deep space, far from any sufficent source of gravity. Such observers are the privileged ("inertial") observers Einstein described in his theory of special relativity: observers for whom light travels along straight lines at constant speed.

Locally, in his inertial frame of reference, he is not accelerating.

Why then can you not comprehend that free-fall has acceleration?  Do you think the word "fall" is just in there to look good? 

People evolved on the ground, as did language.  Something falling from the sky onto your head seems to suggest that it is that object that is accelerating rather than you, but relativity has a habit of showing us how counter-intuitive reality is.  In any meaningful sense, by standing on the ground it is us accelerating, not someone 'falling' from the sky above.

What co-ordinate system? Whose rulers and clocks are we using? In any system, only one of them is accelerating in any meaningful sense, and that is the guy on the ground.

  A stationary one.  Please state your definition of free-fall.   

As for your second point - what are you talking about? Where did the 'both poles of the Earth' thing come from?

It was a simple thought experiment, the earth cannot accelerate in two directions at once.   

Yes, they come factory-calibrated to read zero at sea level.  See wiki:

Quote from: http://en.wikipedia.org/wiki/Accelerometer

As a consequence, the output of an accelerometer has an offset due to local gravity. This means that, perhaps counter-intuitively, an accelerometer at rest on the earth's surface will actually indicate 1 g along the vertical axis. To obtain the acceleration due to motion alone, this offset must be subtracted.

I said force not acceleration. 

Quote from: Matrix on September 22, 2008, 10:24:13 AM

What co-ordinate system?

A stationary one.

More genius for the monster fail thread!

Why then can you not comprehend that free-fall has acceleration?   

He is accelerating relative to an observer on earth in a non-inertial for, but in his local for he is following a straight line geodesic at a uniform speed.

Locally, in his inertial frame of reference, he is not accelerating.

Locally he cannot tell he is accelerating as everything else is accelerating in the exact same way.

Quote from: sokarul on September 22, 2008, 10:28:12 AM

Quote from: Matrix on September 22, 2008, 10:24:13 AM

What co-ordinate system?

A stationary one.

More genius for the monster fail thread!

Why do you keep dodging my questions? 

Quote from: Osama  bin Laden on September 22, 2008, 10:29:25 AM

Quote from: sokarul on September 22, 2008, 10:28:12 AM

Quote from: Matrix on September 22, 2008, 10:24:13 AM

What co-ordinate system?

A stationary one.

More genius for the monster fail thread!

Why do you keep dodging my questions? 

Why do you keep failing so hard?

Why do you keep failing so hard?

You are not answering the questions because you know you are the one thats failing. 

A stationary one.  Please state your definition of free-fall.

I define free-fall to be the frame of reference defined by motion along a geodesic, which can be achieved only by the application of no external forces on the observer.

It was a simple thought experiment, the earth cannot accelerate in two directions at once.

The Earth itself follows a geodesic (neglecting solar wind et al.), but that says nothing for objects on it's surface.

Quote

Yes, they come factory-calibrated to read zero at sea level.  See wiki:

Quote from: http://en.wikipedia.org/wiki/Accelerometer

As a consequence, the output of an accelerometer has an offset due to local gravity. This means that, perhaps counter-intuitively, an accelerometer at rest on the earth's surface will actually indicate 1 g along the vertical axis. To obtain the acceleration due to motion alone, this offset must be subtracted.

I said force not acceleration. 

If you read the wikipedia article you will find that force is intimately related to accelerometers, namely that an accelerometer is merely a calibrated force meter (very basically, it's a spring with a test mass attached).

Quote from: sokarul on September 22, 2008, 10:28:12 AM

A stationary one.  Please state your definition of free-fall.

I define free-fall to be the frame of reference defined by motion along a geodesic, which can be achieved only by the application of no external forces on the observer.

Ok, now what is motion along geodesic?  Never mind, I will just tell you, I don't have all day to run around in circles.  It's acceleration without a force. 

The Earth itself follows a geodesic (neglecting solar wind et al.), but that says nothing for objects on it's surface.

You missed the point. 

If you read the wikipedia article you will find that force is intimately related to accelerometers, namely that an accelerometer is merely a calibrated force meter (very basically, it's a spring with a test mass attached).

I know what acceleration is. Now back to simple force scales.  You never used one?   

I still loved the time when he claimed inertial motion is different from free-fall, and then suddenly reversed that. Too bad, the OP deleted the thread to save his sorry ass.

From physics 301 regarding free-falling reference frames:

Now consider you are in an elevator which is in free-fall in the gravity of the Earth. There is an apple
sitting on the floor next to you. During your free-fall, you will not feel the floor pushing up on you since you
have the same velocity and acceleration as the elevator. You will also see the apple is stationary with respect to
you. In other words, it has no acceleration within the local reference frame associated with your motion. In
other words, your local reference frame (the elevator) shows no evidence of forces acting. So gravity is
undetectable by observing relative motion of these two bodies. Acceleration due do gravity is undetectable by a
frame that is accelerated in time with physical objects subject to gravity & no other forces.

-“Free fall reference frame”: cannot, however, be extended arbitrarily far through space & time since the
strength of gravity will change.

- “With respect to a free-falling frame of reference, material bodies will be unaccelerated if they are free
from non-gravitational forces.”

- This is the same as inertial frames of reference when there is no gravity present.
When there is NO gravity present, a frame of reference is extensible (can be extended in space)

- But when gravity is present, we only can construct a local frame of reference. The reference frame
cannot be extended arbitrarily far thru space and time since the strength of gravity will change with
position.

From physics 301 regarding free-falling reference frames:

Now consider you are in an elevator which is in free-fall in the gravity of the Earth. There is an apple
sitting on the floor next to you. During your free-fall, you will not feel the floor pushing up on you since you
have the same velocity and acceleration as the elevator. You will also see the apple is stationary with respect to
you. In other words, it has no acceleration within the local reference frame associated with your motion. In
other words, your local reference frame (the elevator) shows no evidence of forces acting. So gravity is
undetectable by observing relative motion of these two bodies. Acceleration due do gravity is undetectable by a
frame that is accelerated in time with physical objects subject to gravity & no other forces.

-“Free fall reference frame”: cannot, however, be extended arbitrarily far through space & time since the
strength of gravity will change.

- “With respect to a free-falling frame of reference, material bodies will be unaccelerated if they are free
from non-gravitational forces.”

- This is the same as inertial frames of reference when there is no gravity present.
When there is NO gravity present, a frame of reference is extensible (can be extended in space)

- But when gravity is present, we only can construct a local frame of reference. The reference frame
cannot be extended arbitrarily far thru space and time since the strength of gravity will change with
position.

Do you even understand your own links?
That doesn't even come close to proving me wrong. 

Sokarul, are you sure you weren't drunk when you posted this?

I have seen SR and some GR in college.

I haven't even been formally educated in GR yet and I have only minimal formal education in SR, and I seem to understand what I'm talking about better than you do.

You took the FOR of freefall and then claimed the earth was accelerating up, even though a freefall FOR is clearly accelerating. 

- “With respect to a free-falling frame of reference, material bodies will be unaccelerated if they are free
from non-gravitational forces.”

How did it not?

Well now that you guys have tried to derail the thread to hide your ignorance I will leave for class.

Sokarul, are you sure you weren't drunk when you posted this?

Quote from: sokarul on August 17, 2008, 01:05:48 PM

I have seen SR and some GR in college.

I haven't even been formally educated in GR yet and I have only minimal formal education in SR, and I seem to understand what I'm talking about better than you do.

No, you don't.  It is clear that you and they do not know what they are talking about.  I will show you guys once again why you are wrong tonight.

Quote from: sokarul on September 22, 2008, 09:52:04 AM

You took the FOR of freefall and then claimed the earth was accelerating up, even though a freefall FOR is clearly accelerating. 

Quote

- “With respect to a free-falling frame of reference, material bodies will be unaccelerated if they are free
from non-gravitational forces.”

How did it not?

The link is stating part of the EP.  "With respect" is not the same as "is".  It is comparing two things.  Nothing there says acceleration is not present in free-fall.   

Nothing there says acceleration is not present in free-fall.   

And with that, I am done.  It's not worth the effort anymore.

Quote from: sokarul on September 22, 2008, 10:52:50 AM

Nothing there says acceleration is not present in free-fall.   

And with that, I am done.  It's not worth the effort anymore.

I gave up about half an hour ago. I'm just posting in this thread for the lulz now.

I don't get it. I've tried everything... hmm... OK actually there's one more possibility.  Sokarul, let's try a little experiment to determine what we should use as this elusive 'stationary frame' you mentioned.  In the following image are 5 people in boxes equipped with accelerometers (we're only concerned with the magnitude of their readouts for now).  Which of them would you consider to be in your 'stationary frame'? (I will fill in some further details about the scene afterwards).

Love the XKCD-esque drawing, Matrix.

Quote from: sokarul on September 22, 2008, 10:52:50 AM

Nothing there says acceleration is not present in free-fall.   

And with that, I am done.  It's not worth the effort anymore.

Good, you saw your error. 

Einstein did not get famous by stating the obvious. 

I gave up about half an hour ago. I'm just posting in this thread for the lulz now.

Yeah we all saw you ignore questions. 

I don't get it. I've tried everything... hmm... OK actually there's one more possibility.  Sokarul, let's try a little experiment to determine what we should use as this elusive 'stationary frame' you mentioned.  In the following image are 5 people in boxes equipped with accelerometers (we're only concerned with the magnitude of their readouts for now).  Which of them would you consider to be in your 'stationary frame'? (I will fill in some further details about the scene afterwards).

There is not enough information given. 
While I wait for you to post the rest of the information, you can read this.

http://en.wikipedia.org/wiki/Gravitational_acceleration

While I wait for you to post the rest of the information, you can read this.

Does anybody else notice anything wrong with this sentence?

Ok now for the original post.

Part of the equivalences principle says a closed box in free-fall will yield the same results as a closed box in outer space with no gravitation present. Now why would that be important if the box in free-fall wasn't doing anything?  It would be like saying 5=5.  That's not going to get you famous.

Objects accelerate due to gravitation.  Since there is no force present, no force is felt, thus free-fall is still inertial.  That is why accelerating free-fall FOR are inertial and accelerating.  This is the weak equivalence principle. 

The weak equivalence principle

The weak equivalence principle, also known as the universality of free fall:

    The trajectory of a falling test body depends only on its initial position and velocity, and is independent of its composition.

or

    All bodies at the same spacetime point in a given gravitational field will undergo the same acceleration.     

Now, where is the evidence that says Einstein was wrong?

Quote from: sokarul on September 22, 2008, 01:15:47 PM

While I wait for you to post the rest of the information, you can read this.

Does anybody else notice anything wrong with this sentence?

yes

Part of the equivalences principle says a closed box in free-fall will yield the same results as a closed box in outer space with no gravitation present. Now why would that be important if the box in free-fall wasn't doing anything?  It would be like saying 5=5.  That's not going to get you famous.

Saying that 1 = -1 might, though.