# Clarification on Gravity

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#### sokarul

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##### Re: Clarification on Gravity
« Reply #750 on: September 23, 2008, 04:46:29 PM »
I guess I have to go along with that people A and E are in local FORes. I'm kinda thinking they are not though.

To be clear at the risk of sounding pedantic, all 5 are in local frames of reference, but only A and E are inertial. You are completely correct in that regard - the two are completely equivalent and no experiment either could do would tell them otherwise.
Orbit is not local.

The problem with saying a guy on the surface of a planet is accelerating upwards is because the only reason he feels a force is because he is being accelerated downwards into the planet so a normal force arises.  That is the only reason why he feels a force.  If he had no acceleration due to gravitation then he would feel no counter-force.

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This reveals the nature of the problem here, I think.  The problem as I see it is that (correct me if I'm wrong) you're saying that the guy on the planet can't just feel a force upwards with no apparent cause - something has to be 'pulling' him the other way.  This is, in the sense that we established before, incorrect - the guy could have 'fallen', by which I mean experienced no forces, to the surface of the planet without knowing it (accelerometer reads zero until contact) and as such the 'motion' caused by gravitation is meaningless - after all, motion itself is only a meaningful concept when two observers tell each other how they appear to be respectively moving.
No, I am still saying it's wrong to say someone on the earths surface has a net acceleration of anything other than zero.  I am also saying it's wrong to say the earth accelerates up to meet a skydiver.

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This is no different to a guy sitting in a rocket until the engine is lit - a sudden and inexplicable increase in force which could be either due to gravitation or to thrust, with no oway to distinguish between them.
Yes as long as you are local.
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We have to be clear that gravitation does not cause acceleration - gravitation defines the framework that we measure acceleration in - that is, the geometry of space-time.
You put to objects with gravitational fields next to each other they will accelerate to each other. Gravitation is word that people use to define the reason why objects attract each other.  This attraction causes acceleration.

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Without wanting to sound patronising, when Einstein wrote E = mc2 he was essentially writing "5 = 5" - the fact that until relativity came along the link between '5' and '5' written in different colour inks (for want of a better analogy) wasn't made explicitly is a testament to his genius for conciseness.
He did not state the obvious.
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As for the principle of equivalence, the same thing can be said there - by stating that again '5' is the same as '5' in any meaningful sense, he revealed a deep insight into the nature of space-time.  If you are in free-fall, you experience no local gravitation (even next to a black hole).  If you are accelerating, you essentially distort your local space-time.  Obviously the cause of the two is different and must be explained as such, but their observable effects are indistinguishable and must be treated as such.
Einstein did not want to say 5=5, he wanted to say 1+4 =2+3.  He took different ideas and related them, he did not take the something and relate it to itself.
Now once again, I know what the equivalence principle is and I know it doesn't take something and relate it to itself.  You have to get over the fact that two things can be equal and not be the same thing.
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#### Jack

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##### Re: Clarification on Gravity
« Reply #751 on: September 23, 2008, 11:45:51 PM »
You have to get over the fact that two things can be equal and not be the same thing.
Right, like 1 = -1.

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#### Mattrix

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##### Re: Clarification on Gravity
« Reply #752 on: September 24, 2008, 12:44:07 AM »
Orbit is not local.

What would be your definition of 'local'? I was merely referring to making measurements at a spacetime point to avoid any arguments over tidal forces and so on.

No, I am still saying it's wrong to say someone on the earths surface has a net acceleration of anything other than zero.  I am also saying it's wrong to say the earth accelerates up to meet a skydiver.

Sadly both of those positions are incorrect, for reasons we have already agreed upon.  It may not make sense to someone standing on the ground to tell them that they are accelerating upwards, but that is exactly what is happening. They are constantly deviating from a geodesic in space-time because (let's forget the Earth's own motion for a moment) that path goes down through the ground beneath them.

It is not that the geodesic is a bungee cord pulling them down, but more that it is the path they would follow if the Earth wasn't in the way.  There is no difference between that case and them hovering above the ground in a rocket, or flying in a rocket in deep space.  All 3 observers measure the same force and all of them agree they are accelerating upwards.

In that sense, the skydiver and the person on the ground could agree that the Earth accelerated up to meet them, because that is what the only sensible coordinate system says happened.  If you choose to constantly reset the ground to be 'zero' in your co-ordinate system then of course you will get a different result, but that co-ordinate system is a non-inertial one.

You put to objects with gravitational fields next to each other they will accelerate to each other. Gravitation is word that people use to define the reason why objects attract each other.  This attraction causes acceleration.

To you, the external observer, yes - the objects appear to accelerate towards each other because you have defined a co-ordinate system according to your own frame (which may or may not be inertial). If you formulate your observations in an inertial frame you will see that neither object accelerated, but that the distance between them decreased because they were both at rest on geodesics.  This is not the same as acceleration, which would have caused them to deviate from those geodesics.

He did not state the obvious.

I would agree - he stated something profound and made it obvious, which is why he was a genius.

Einstein did not want to say 5=5, he wanted to say 1+4 =2+3.  He took different ideas and related them, he did not take the something and relate it to itself.
Now once again, I know what the equivalence principle is and I know it doesn't take something and relate it to itself.  You have to get over the fact that two things can be equal and not be the same thing.

The EP tells us how to treat co-ordinates and forces in a sensible way. The implication is that the only sensible co-ordinate system is an inertial one, in which case the observer on the surface of the Earth is accelerating upwards always.  Of course the Earth isn't exploding outwards, and that is because the co-ordinate system we choose says that as it accelerates outwards, we move our co-ordinates inwards along geodesics.
« Last Edit: September 24, 2008, 12:49:05 AM by Mattrix »
the earth spins, and as it does, the earth heats up, and cools down respectively.

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#### Fletch

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##### Re: Clarification on Gravity
« Reply #753 on: September 24, 2008, 02:06:13 AM »
I still want to know how he plans to use Atomic Clocks to prove the surface of the earth isn't accelerating.

Not talking about the current topic but acceleration in general, atomic clocks are used to show time dilation.  Did you look up the twin paradox and the answer?
I didn't see you ask me to, but I'm aware of the concept of time slowing as you increase your speed. How would you use that principle?
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#### divito the truthist

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##### Re: Clarification on Gravity
« Reply #754 on: September 24, 2008, 05:13:26 AM »
What would be your definition of 'local'?

Not sure what his is, but:

"The important point is that special relativity applies only locally, that is, in your nearby vicinity. In your local neighborhood (e.g., the solar system), special relativity holds, as we confirm with our space probes. But globally (e.g., on cosmological scales involving the universe) we must use general relativity instead." - Michio Kaku
« Last Edit: September 24, 2008, 05:20:17 AM by divito the truthist »
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#### sokarul

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##### Re: Clarification on Gravity
« Reply #755 on: September 24, 2008, 09:57:56 AM »

What would be your definition of 'local'? I was merely referring to making measurements at a spacetime point to avoid any arguments over tidal forces and so on.
One where the EP applies.
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Sadly both of those positions are incorrect, for reasons we have already agreed upon.  It may not make sense to someone standing on the ground to tell them that they are accelerating upwards, but that is exactly what is happening. They are constantly deviating from a geodesic in space-time because (let's forget the Earth's own motion for a moment) that path goes down through the ground beneath them.
Those positions are not incorrect.  Their paths would go through the ground so going through the ground is up?  There net acceleration is still zero.

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It is not that the geodesic is a bungee cord pulling them down, but more that it is the path they would follow if the Earth wasn't in the way.  There is no difference between that case and them hovering above the ground in a rocket, or flying in a rocket in deep space.  All 3 observers measure the same force and all of them agree they are accelerating upwards.
I can measure the same force whether I am driving my car or a Ferrari.  So I drive a Ferrari?  A person on the ground is undergoing a constant physical acceleration downwards which causes a normal force to be felt.

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In that sense, the skydiver and the person on the ground could agree that the Earth accelerated up to meet them, because that is what the only sensible coordinate system says happened.  If you choose to constantly reset the ground to be 'zero' in your co-ordinate system then of course you will get a different result, but that co-ordinate system is a non-inertial one.
The sensible coordinate system you speak of is by definition, accelerating.

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To you, the external observer, yes - the objects appear to accelerate towards each other because you have defined a co-ordinate system according to your own frame (which may or may not be inertial). If you formulate your observations in an inertial frame you will see that neither object accelerated, but that the distance between them decreased because they were both at rest on geodesics.  This is not the same as acceleration, which would have caused them to deviate from those geodesics.
Objects traveling in locally straight paths can still accelerate.

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The EP tells us how to treat co-ordinates and forces in a sensible way. The implication is that the only sensible co-ordinate system is an inertial one, in which case the observer on the surface of the Earth is accelerating upwards always.
Free-fall is inertial motion even thought there is acceleration present.
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#### Robbyj

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##### Re: Clarification on Gravity
« Reply #756 on: September 24, 2008, 11:32:14 AM »
All six of those statements are incorrect.
Why justify an illegitimate attack with a legitimate response?

#### Parsifal

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##### Re: Clarification on Gravity
« Reply #757 on: September 24, 2008, 11:42:03 AM »
All six of those statements are incorrect.

Did you expect something else? Just a reminder, you're talking to Sokarul.
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#### Dr Matrix

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##### Re: Clarification on Gravity
« Reply #758 on: September 24, 2008, 02:20:05 PM »
I was going to go through them all one by one, but I'd just end up repeating myself. Again.
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#### Parsifal

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##### Re: Clarification on Gravity
« Reply #759 on: September 24, 2008, 02:31:29 PM »
I was going to go through them all one by one, but I'd just end up repeating myself. Again.

Yes, you get that a lot with Sokarul. He doesn't listen, just restates the same blatantly false information, and in the most extreme cases accuses you of saying the stupid things that, in actuality, came from him.
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#### sokarul

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##### Re: Clarification on Gravity
« Reply #760 on: September 24, 2008, 08:44:16 PM »
All six of those statements are incorrect.
Nope
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#### Dr Matrix

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##### Re: Clarification on Gravity
« Reply #761 on: September 25, 2008, 02:08:07 AM »
What would be your definition of 'local'? I was merely referring to making measurements at a spacetime point to avoid any arguments over tidal forces and so on.
One where the EP applies.

That makes no sense at all - 'local' in the most extreme definition refers to a single spacetime point, which must see a Minkowski metric. That is why GR guys get so pissed off with people like Teyssandier who do work implying that you can measure your absolute gravitational potential locally.

For the purposes of this discussion, 'local' means 'human scale', where we assume that we deal with fields weak enough to neglect tidal forces.  That's basically any field away from an event horizon or objects moving at relativistic velocity compared to the observer.

Those positions are not incorrect.  Their paths would go through the ground so going through the ground is up?  There net acceleration is still zero.

Their spatial paths go nowhere - that's the whole point of a geodesic.  Their space-time path (the geodesic) goes through the ground, which means that when they come into contact with the ground they experience an acceleration which causes them to leave their geodesic (contact acceleration). The ground is accelerating them upwards. If they are in an inertial reference frame then they're not going anywhere in any meaningful sense, it is the ground that accelerates up to meet them.  That doesn't make much sense from the perspective of you standing on the ground, but nature doesn't give a shit what makes more sense to you, that's just the way it is. Get over it.

I can measure the same force whether I am driving my car or a Ferrari.  So I drive a Ferrari?  A person on the ground is undergoing a constant physical acceleration downwards which causes a normal force to be felt.

No. A person experiences a normal force and hence a normal acceleration as a result of being in contact with a giant mass of rock. There is no 'natural acceleration' since if there was then we would be able to measure it while in free-fall (an inertial frame) with an accelerometer.

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In that sense, the skydiver and the person on the ground could agree that the Earth accelerated up to meet them, because that is what the only sensible coordinate system says happened.  If you choose to constantly reset the ground to be 'zero' in your co-ordinate system then of course you will get a different result, but that co-ordinate system is a non-inertial one.
The sensible coordinate system you speak of is by definition, accelerating.

Surely a sensible co-ordinate system is one in which 'rest' and hence the zeroes of the axes are defined when no forces are felt on the observer? That frame would be free fall, so if the observer sits at zero until he feels a force on him, then it is the guy on the surface of the Earth that is accelerating, not the free-faller.  This makes a lot more sense than setting some arbitrary acceleration (g) to be 'zero'.

Objects traveling in locally straight paths can still accelerate.

No, they can't.  A locally straight path is a geodesic, which follows a straight path through space-time.

Free-fall is inertial motion even thought there is acceleration present.

Free-fall is an inertial frame.  You experience no acceleration in an inertial frame.  Inertial frames can not accelerate.  You are thinking in terms of a non-inertial co-ordinate system - this thinking will lead you to draw incorrect conclusions, as has been demonstrated already.
Quote from: Arthur Schopenhauer
All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident.

#### sokarul

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##### Re: Clarification on Gravity
« Reply #762 on: September 25, 2008, 10:09:01 AM »

That makes no sense at all - 'local' in the most extreme definition refers to a single spacetime point, which must see a Minkowski metric. That is why GR guys get so pissed off with people like Teyssandier who do work implying that you can measure your absolute gravitational potential locally.

For the purposes of this discussion, 'local' means 'human scale', where we assume that we deal with fields weak enough to neglect tidal forces.  That's basically any field away from an event horizon or objects moving at relativistic velocity compared to the observer.
No, it made sense, that's why you repeated it.  So you can see why orbit is not local.

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Their spatial paths go nowhere - that's the whole point of a geodesic.  Their space-time path (the geodesic) goes through the ground, which means that when they come into contact with the ground they experience an acceleration which causes them to leave their geodesic (contact acceleration). The ground is accelerating them upwards. If they are in an inertial reference frame then they're not going anywhere in any meaningful sense, it is the ground that accelerates up to meet them.  That doesn't make much sense from the perspective of you standing on the ground, but nature doesn't give a shit what makes more sense to you, that's just the way it is. Get over it.
Why would I get over being right?  So the ground accelerates them, then why do they not leave the ground?

I can measure the same force whether I am driving my car or a Ferrari.  So I drive a Ferrari?  A person on the ground is undergoing a constant physical acceleration downwards which causes a normal force to be felt.

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No. A person experiences a normal force and hence a normal acceleration as a result of being in contact with a giant mass of rock. There is no 'natural acceleration' since if there was then we would be able to measure it while in free-fall (an inertial frame) with an accelerometer.
Unless of course the acceleration doesn't come from a force.  You know, what Einstein said.

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Surely a sensible co-ordinate system is one in which 'rest' and hence the zeroes of the axes are defined when no forces are felt on the observer? That frame would be free fall, so if the observer sits at zero until he feels a force on him, then it is the guy on the surface of the Earth that is accelerating, not the free-faller.  This makes a lot more sense than setting some arbitrary acceleration (g) to be 'zero'.
This doesn't make more sense.  Wait for it.

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No, they can't.  A locally straight path is a geodesic, which follows a straight path through space-time.
Yes locally straight paths through curved spacetime.

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Free-fall is an inertial frame.  You experience no acceleration in an inertial frame.  Inertial frames can not accelerate.
Agreed
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You are thinking in terms of a non-inertial co-ordinate system - this thinking will lead you to draw incorrect conclusions, as has been demonstrated already.
no

How does NASA use other planets and the sun to Slingshot space probes?  You ever heard of them doing that?  A probe will enter close to the sun as to start curving and accelerating by gravitation. But the angle at which it enters is the right angle as to no hit the sun or to get stuck in an orbit.  The probe exits with a very high velocity, much higher than when it entered.  Now tell me, how does that work?  The probe never feels a force.
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#### Dr Matrix

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##### Re: Clarification on Gravity
« Reply #763 on: September 25, 2008, 10:11:20 AM »
I'll deal with the rest later as I'm in a hurry, but gravitational slingshot = conservation of momentum.  Gravitation acts as a mediator to transfer momentum from the planet to the probe.
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#### sokarul

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##### Re: Clarification on Gravity
« Reply #764 on: September 25, 2008, 10:14:39 AM »
I'll deal with the rest later as I'm in a hurry, but gravitational slingshot = conservation of momentum.  Gravitation acts as a mediator to transfer momentum from the planet to the probe.
hahahaha, I really did lol at that one.
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#### Parsifal

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##### Re: Clarification on Gravity
« Reply #765 on: September 25, 2008, 12:49:05 PM »
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Their spatial paths go nowhere - that's the whole point of a geodesic.  Their space-time path (the geodesic) goes through the ground, which means that when they come into contact with the ground they experience an acceleration which causes them to leave their geodesic (contact acceleration). The ground is accelerating them upwards. If they are in an inertial reference frame then they're not going anywhere in any meaningful sense, it is the ground that accelerates up to meet them.  That doesn't make much sense from the perspective of you standing on the ground, but nature doesn't give a shit what makes more sense to you, that's just the way it is. Get over it.
Why would I get over being right?  So the ground accelerates them, then why do they not leave the ground?

Lol.

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No, they can't.  A locally straight path is a geodesic, which follows a straight path through space-time.
Yes locally straight paths through curved spacetime.

Exactly.

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Free-fall is an inertial frame.  You experience no acceleration in an inertial frame.  Inertial frames can not accelerate.
Agreed

So you've finally accepted that we're right. Good to hear.
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#### sokarul

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##### Re: Clarification on Gravity
« Reply #766 on: September 25, 2008, 12:54:17 PM »

So you've finally accepted that we're right. Good to hear.
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How does NASA use other planets and the sun to Slingshot space probes?  You ever heard of them doing that?  A probe will enter close to the sun as to start curving and accelerating by gravitation. But the angle at which it enters is the right angle as to no hit the sun or to get stuck in an orbit.  The probe exits with a very high velocity, much higher than when it entered.  Now tell me, how does that work?  The probe never feels a force.
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#### Parsifal

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##### Re: Clarification on Gravity
« Reply #767 on: September 25, 2008, 01:01:16 PM »
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How does NASA use other planets and the sun to Slingshot space probes?  You ever heard of them doing that?  A probe will enter close to the sun as to start curving and accelerating by gravitation. But the angle at which it enters is the right angle as to no hit the sun or to get stuck in an orbit.  The probe exits with a very high velocity, much higher than when it entered.  Now tell me, how does that work?  The probe never feels a force.

Try imagining spacetime as a three-dimensional manifestation of two-dimensional space and one-dimensional time. An increase in velocity may be seen in this construct as travelling further through space per unit time. Now, tell me you can't see why this may be achieved through curvature of such spacetime.
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#### sokarul

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##### Re: Clarification on Gravity
« Reply #768 on: September 25, 2008, 01:09:11 PM »
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How does NASA use other planets and the sun to Slingshot space probes?  You ever heard of them doing that?  A probe will enter close to the sun as to start curving and accelerating by gravitation. But the angle at which it enters is the right angle as to no hit the sun or to get stuck in an orbit.  The probe exits with a very high velocity, much higher than when it entered.  Now tell me, how does that work?  The probe never feels a force.

Try imagining spacetime as a three-dimensional manifestation of two-dimensional space and one-dimensional time. An increase in velocity may be seen in this construct as travelling further through space per unit time. Now, tell me you can't see why this may be achieved through curvature of such spacetime.
I can and that is why the acceleration or an "increase in velocity" comes about.
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#### Parsifal

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##### Re: Clarification on Gravity
« Reply #769 on: September 25, 2008, 01:14:53 PM »
I can and that is why the acceleration or an "increase in velocity" comes about.

There is no acceleration. It's just that you're observing the spacecraft from your frame of reference. In its frame of reference, it is the rest of the Universe that has sped up. Ergo, no acceleration.
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#### sokarul

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##### Re: Clarification on Gravity
« Reply #770 on: September 25, 2008, 01:27:08 PM »

There is no acceleration. It's just that you're observing the spacecraft from your frame of reference. In its frame of reference, it is the rest of the Universe that has sped up. Ergo, no acceleration.
Yes from my inertial FOR I see the probe accelerate.  The probes FOR does not detect acceleration as gravitation is not a force.  But the probe most certainly does speed up as it will get to me at a faster velocity than when it left and I sure didn't feel an acceleration.  So the only way you can explain the probe speeding up is to claim the entire universe accelerated instead?
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#### Parsifal

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##### Re: Clarification on Gravity
« Reply #771 on: September 25, 2008, 01:31:12 PM »
Yes from my inertial FOR I see the probe accelerate.  The probes FOR does not detect acceleration as gravitation is not a force.  But the probe most certainly does speed up as it will get to me at a faster velocity than when it left and I sure didn't feel an acceleration.  So the only way you can explain the probe speeding up is to claim the entire universe accelerated instead?

No. Nobody has accelerated. This is not 1687, some of us have moved into the 21st century.
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#### sokarul

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##### Re: Clarification on Gravity
« Reply #772 on: September 25, 2008, 01:35:34 PM »
Yes from my inertial FOR I see the probe accelerate.  The probes FOR does not detect acceleration as gravitation is not a force.  But the probe most certainly does speed up as it will get to me at a faster velocity than when it left and I sure didn't feel an acceleration.  So the only way you can explain the probe speeding up is to claim the entire universe accelerated instead?

No. Nobody has accelerated. This is not 1687, some of us have moved into the 21st century.
Velocity cannot increase with out acceleration.  So are you going to explain why the probe leaves with x velocity and comes back with y velocity where x<y?
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#### Parsifal

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##### Re: Clarification on Gravity
« Reply #773 on: September 25, 2008, 01:37:14 PM »
Velocity cannot increase with out acceleration.  So are you going to explain why the probe leaves with x velocity and comes back with y velocity where x<y?

See: General Relativity.
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#### sokarul

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##### Re: Clarification on Gravity
« Reply #774 on: September 25, 2008, 01:39:36 PM »
Velocity cannot increase with out acceleration.  So are you going to explain why the probe leaves with x velocity and comes back with y velocity where x<y?

See: General Relativity.

I did and you know what it said?

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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.

So now we can move past this gravitation doesn't causes acceleration when we know it can and we use it all the time.
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#### Parsifal

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##### Re: Clarification on Gravity
« Reply #775 on: September 25, 2008, 01:47:46 PM »
Velocity cannot increase with out acceleration.  So are you going to explain why the probe leaves with x velocity and comes back with y velocity where x<y?

See: General Relativity.

I did and you know what it said?

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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.

So now we can move past this gravitation doesn't causes acceleration when we know it can and we use it all the time.

I agree with the statement in bold. It doesn't make your point any stronger.
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#### sokarul

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##### Re: Clarification on Gravity
« Reply #776 on: September 25, 2008, 01:49:51 PM »
Velocity cannot increase with out acceleration.  So are you going to explain why the probe leaves with x velocity and comes back with y velocity where x<y?

See: General Relativity.

I did and you know what it said?

Quote
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.

So now we can move past this gravitation doesn't causes acceleration when we know it can and we use it all the time.

I agree with the statement in bold. It doesn't make your point any stronger.
It does and you still haven't explained "why the probe leaves with x velocity and comes back with y velocity where x<y".
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#### Parsifal

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##### Re: Clarification on Gravity
« Reply #777 on: September 25, 2008, 01:53:50 PM »
It does and you still haven't explained "why the probe leaves with x velocity and comes back with y velocity where x<y".

It doesn't, and I have. See: General Relativity.
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#### Robbyj

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##### Re: Clarification on Gravity
« Reply #778 on: September 25, 2008, 01:56:01 PM »
I'll deal with the rest later as I'm in a hurry, but gravitational slingshot = conservation of momentum.  Gravitation acts as a mediator to transfer momentum from the planet to the probe.

He did.

http://www.mathpages.com/home/kmath114.htm
Why justify an illegitimate attack with a legitimate response?

#### sokarul

• 18883
• Extra Racist
##### Re: Clarification on Gravity
« Reply #779 on: September 25, 2008, 01:56:53 PM »
It does and you still haven't explained "why the probe leaves with x velocity and comes back with y velocity where x<y".

It doesn't, and I have. See: General Relativity.
See: Space exploration.

It is done all the time to raise the velocity of space probes.
Now are you going to refute anything or are you going to keep cowering out?  I have a lab report to write and if you aren't going to back up anything you say I going to go write it.
ANNIHILATOR OF  SHIFTER

It's no slur if it's fact.