or Connect
AppleInsider › Forums › Mac Hardware › Future Apple Hardware › CES: Corning Gorilla Glass 2 is 0.8mm thick, withstands 121 pounds of pressure
New Posts  All Forums:Forum Nav:

CES: Corning Gorilla Glass 2 is 0.8mm thick, withstands 121 pounds of pressure - Page 4

post #121 of 147
Quote:
Originally Posted by SolipsismX View Post

I love this forum because of the very diverse and in-depth conversations(arguments) that often occur. I am a member of many forums yet this only happens on AI's forum. It's great! I certainly learned something about gravity from this thread.

I agree. I think it is a reflection of the diverse range of expertise of posters here. I wish I could contribute more to the forum's core subject areas, but mostly I end up just listening and learning.
post #122 of 147
Quote:
Originally Posted by majjo View Post

Oh god, this thread is still going?

For good reason, I think. Besies, it's not the only thread lingering on. Check here
How to enter the Apple logo  on iOS:
/Settings/Keyboard/Shortcut and paste in  which you copied from an email draft or a note. Screendump
Reply
How to enter the Apple logo  on iOS:
/Settings/Keyboard/Shortcut and paste in  which you copied from an email draft or a note. Screendump
Reply
post #123 of 147
Quote:
Originally Posted by majjo View Post

Dont get me wrong, thats great. It just feels like in this case, theyre starting to argue for the sake of arguing (at least thats the impression i get from the last few posts). Im glad you learned something from it tho.

I'd like to claim that's not the case - I certainly hope I'm not doing that. From my perspective, differences in opinion are just fine and I have no problem letting them go, but fundamental arguments on fact or reasoning (or both in this case), especially in my own field, will keep me coming back until a resolution is reached, or until it becomes clear that a resolution is not the goal of the discussion.
post #124 of 147
Quote:
Originally Posted by majjo View Post

From my perspective, he already stated that the earth budges out at the equator. So he already implies that he understands that there is a centripetal force; unless he thinks theres a different explaination for the fact that the earth is not a sphere (which i would be interested to hear). So when he then throws out the "no such thing as centripetal force" it feels like hes just doing it to continue the argument.

Yes - I missed that point in your previous post, and of course you are correct - that is the reason for the oblate shape. However, I'm not sure that he realizes that. I also don't think he was denying the existence of centripetal acceleration just to prolong the argument, because that assertion is too demonstrably wrong - I think he just got confused somehow. Pretty determinedly confused I admit.

Quote:
Originally Posted by majjo View Post

With regards to the original confusion, i really think we should just set a hard definition of gravitational force, at least for this thread. Because it can get confusing when different posters refer to gravity to mean different things. Personally i perfer gravity to refer to solely the force from the fact that there is mass i.e. GMm/r^2. And we refer to the total force @ the earth's surface as just that, or F(es).

No problem there, except, as I mentioned earlier, the original discussion was about the reduction in weight of an object at the equator relative to the poles. A discussion of just the gravitational field strength does not fully address that effect, because it is partly due to the oblate shape of the earth, so that you are farther from the center of mass of the earth when at the equator (and the field strength is inversely proportional to r²), and partly due to the centripetal acceleration at the equator. In fact the effects are almost equal.
post #125 of 147
And just to nip in the bud any further question about centripetal acceleration, here is an a priori derivation of centripetal acceleration for motion in a circle, of radius r, at a constant angular velocity, ω. Working within the formatting options available, omitting the italics for variables convention for readability and using bold face to denote vectors, and using polar coordinates (r, φ) for simplicity, the circular motion is defined in terms of the unit x and y vectors (x, y) by:
r = r(ysinφ + xcosφ) . [1] For constant motion where the angular velocity dφ/dt is ω and φ(t) = ωt, we can write the time-dependent position vector r(t) as
r(t) = r(ysinωt + xcosωt) . [2] The velocity vector, v(t) is dr/dt, so differentiating [2] w.r.t. t we get
v(t) = ωr(ycosωt - xsinωt) . [3] The acceleration vector, a(t), is dv/dt, so differentiating again w.r.t. t, we get
a(t) = - ω²r(ysinωt + xcosωt) . [4] Substituting r(t) for r(ysinωt + xcosωt) from [1] in the R.H.S. of [4] we get
a(t) = - ω²r(t) , [5] which is a centripetal acceleration of constant magnitude ω²r parallel to the r vector and directed towards (hence the -ve sign) the center of the circle.
post #126 of 147
Quote:
Originally Posted by SolipsismX View Post

So we're arguing the very clear definition of gravity vs the effects of gravity of a given planetary body acting upon an object when accounting for effects that would negate the perceived effects of gravity at a given location?

Some mess like that. No reasoning with Wikipedia Physicists.
.
Reply
.
Reply
post #127 of 147
Quote:
Originally Posted by muppetry View Post

And just to nip in the bud any further question about centripetal acceleration, here is an a priori derivation of centripetal acceleration for motion in a circle, of radius r, at a constant angular velocity, ω.

<snipped entirely valid rectangular reference plane cycloid motion derivation>

Which is pretty meaningless because you cannot compare there results of a Cartesian reference frame directly to results of the polar coordinate reference frame. Pick the right reference frame and the problems get simple, pick the wrong one and you spend a lot of effort constructing virtual vectors to balance things out.
.
Reply
.
Reply
post #128 of 147
Quote:
Originally Posted by muppetry View Post

Yes - I missed that point in your previous post, and of course you are correct - that is the reason for the oblate shape. However, I'm not sure that he realizes that. I also don't think he was denying the existence of centripetal acceleration just to prolong the argument, because that assertion is too demonstrably wrong - I think he just got confused somehow. Pretty determinedly confused I admit.

What you keep missing is that centripetal acceleration is not gravity, nor any component of gravity! You can keep on for ages about all kinds of fruffery, but gravity is gravity.

So maybe you should lose the judgements of who thinks what about what causes what and get back to actually getting your basic terminology correct. 'Cause if you can't get the basics right there's no place to go.
.
Reply
.
Reply
post #129 of 147
Quote:
Originally Posted by Hiro View Post

Which is pretty meaningless because you cannot compare there results of a Cartesian reference frame directly to results of the polar coordinate reference frame. Pick the right reference frame and the problems get simple, pick the wrong one and you spend a lot of effort constructing virtual vectors to balance things out.

Bullshit - of course you can. I'm starting to doubt whether you even know the meaning of the technical terms you are throwing around. Do you actually have any physics or mathematics training or are you just trying, unsuccessfully, to pass yourself off as knowledgeable? Observations in any inertial frame of reference are identical - that is the definition of an inertial frame of reference. You are confusing inertial and non-inertial frames of reference. The coordinate system, polar, cartesian, spherical, or other arbitrary system, within an inertial frame of reference is doubly irrelevant. Didn't you know that?

Either way, now you are just obfuscating. Are you, or are you not, still denying the existence of centripetal acceleration for constant circular motion, as you did in post 117 and used as your basis to discard my entire analysis as flawed? Actually don't bother to answer that. You haven't come up with a cogent or accurately reasoned argument since this discussion started, and I can conclude with reasonable certainty that you are not having this discussion in good faith.
post #130 of 147
Quote:
Originally Posted by Hiro View Post

What you keep missing is that centripetal acceleration is not gravity, nor any component of gravity! You can keep on for ages about all kinds of fruffery, but gravity is gravity.

So maybe you should lose the judgements of who thinks what about what causes what and get back to actually getting your basic terminology correct. 'Cause if you can't get the basics right there's no place to go.

No one, as far as I can tell, has been suggesting any such equality - that is another of your diversions. All that was being discussed was the variation in weight of an object with latitude, and its causes - at least until you showed up and derailed the whole subject with your nonsense. Nice work, if that is what you intended.

Basics? What would you know about basics? In this field, virtually nothing, as you keep demonstrating. You haven't engaged technically with a single point that I have made (apart from your lame and flawed attempt to dispute centripetal acceleration) - just dismissed them and then made up seemingly random combinations of technical terms that presumably you are used to passing off as intelligent comment. I'm sorry, but you are a ridiculously frustrating person to argue with.
post #131 of 147
Quote:
Originally Posted by majjo View Post

Explain to me how those two do not contradict each other?

I flipped the words centrifugal and centripetal when responding to a post about centrifugal force reducing the feeling of gravity (@mstones post). A terminology faux pas, but the math was always and still is still correct when you unflip my boo-boo term. How nobody else realized that and mentioned it then this is surprising having a look back at the original post.

We can both agree that @mstones quoted virtual centrifugal force absolutely cannot reduce the force of gravity because it doesn't actually exist, not to mention the fact that the virtual centrifugal vector points in a direction that would add to the apparent gravity vector, contradicting a lessening.

As for the rest, it's simple. Gravity is gravity. I don't care about centripetal forces as a component of gravity, because they aren't -- that has been my whole point. Yes they are why the earth accreted the way is did, but they are NOT a component of gravity. Period. Ever. (combined forces apparent gravity != gravity)

What is so freaking hard about this?
.
Reply
.
Reply
post #132 of 147
Quote:
Originally Posted by Hiro View Post

I flipped the words centrifugal and centripetal when responding to a post about centrifugal force reducing the feeling of gravity (@mstones post). A terminology faux pas, but the math was always and still is still correct when you unflip my boo-boo term. How nobody else realized that and mentioned it then this is surprising having a look back at the original post.

OK this is better. Let's have one more attempt to see if we can reconcile this. I have no problem forgiving a mistyped word, but I was referring to post 117 where you said:

"

Bad assumptions. You are trying to measure from the wrong frame and wrong coordinate system to make that direct comparison. When the frame of reference is the center of rotation of the Earth there is no radial acceleration of an object on it's surface because the Earth is for all intent and purposes rotating at constant velocity (on a scale of billions of years)." You used that to dispute the simple derivation of weight at the equator that I posted in post 116. However, the only assumptions that I made were the equations of motion for simple, constant, circular motion, which I then derived from first principles in post 131.

Further, those equations are independent of the frame of reference, provided that it is an inertial frame of reference. You said "when the frame of reference is the center of rotation of the earth". That would be an inertial frame of reference if it does not rotate with the earth, or a non-inertial frame of reference if it does rotate with the earth. Since you ruled out the existence of centrifugal forces (which only exist in non-inertial rotating frames of reference), I concluded that you too were referring to the non-rotating inertial frame in which my analysis was conducted.

Quote:
Originally Posted by Hiro View Post

We can both agree that @mstones quoted virtual centrifugal force absolutely cannot reduce the force of gravity because it doesn't actually exist, not to mention the fact that the virtual centrifugal vector points in a direction that would add to the apparent gravity vector, contradicting a lessening.

I agree that in the non-rotating, inertial frame of reference of the center of the earth, there is no centrifugal force - but two caveats:
  1. In the rotating, non-inertial frame of reference that is the apparent frame of reference to an observer on the surface of the earth, that force does exist, and the analysis in that frame of reference does produce exactly the same result.
  2. In that non-inertial frame of reference, the centrifugal force vector points outwards (+ve r direction), so it is in opposition to the gravitational force vector that points towards the center of the earth (-ve r direction) and would reduce it.

However, let's stay in the inertial frame of reference where we don't have a centrifugal force.

Quote:
Originally Posted by Hiro View Post

As for the rest, it's simple. Gravity is gravity. I don't care about centripetal forces as a component of gravity, because they aren't -- that has been my whole point. Yes they are why the earth accreted the way is did, but they are NOT a component of gravity. Period. Ever. (combined forces apparent gravity != gravity)

Well yes, I agree, but that's not what you said earlier. And it is not actually that simple, because we were discussing g, our measure of gravity on earth. I made that distinction clear in #116, and yet you still dismissed the analysis as flawed for a bunch of other spurious reasons, most notably claiming (see above) that there was no centripetal acceleration on the equator.

So - let me just restate the argument one more time to make sure that we are not just talking about different things. Maybe you can identify precisely where you disagree, if you still do.

The original discussion was about variation in weight, W, with latitude, where the weight of an object of mass m is defined by
W = mg . [1] From my derivation in #131 that I won't write out again, the centripetal acceleration, a, of an object on the equator, equatorial radius R, angular velocity of rotation ω, is
a = - ω²R . [2] The force, F, exerted on the object due to the gravitational attraction of the earth, mass M, is given by
F = - GMm/R² . [3] W, the weight of the object, is by definition the normal reaction between the object and the surface of the earth, and since the resultant force on the object must equal the product of its mass and acceleration,
F + W = ma . [4] Substituting for W, a and F from [1], [2] and [3] respectively, we get

-

GMm/R² + mg = - ²R . [5] Rearranging, and dividing through by m,
g = GM/R² - ω²R . [6] Writing this more generally than for just on the equator, for latitude L, since we need two different Rs for the two terms on the RHS, one for distance to the center of the earth, R(L), in the first term, and one for distance to the axis of rotation, r(L), in the second term, and take into account that the acceleration vector is normal to the axis of rotation rather than the surface of the earth
g = GM/R² - ω²rsinL , [7] or explicitly writing r = RsinL
g = GM/R² - ω²Rsin²L , [7] OK - so this is the g that we use as acceleration due to gravity on earth - it is what we multiply mass by to get weight, and it is the acceleration towards the earth's surface of an object when dropped - and it varies with latitude, not only because the oblate shape of the earth means that R varies slightly, but also because L goes from 90˚ at the poles to 0˚ at the equator. It is the quantity that was being discussed.

In our inertial frame of reference, no question that it differs from the gravitational field strength (GM/R²) by the centripetal acceleration, as I pointed out in an earlier post when I was trying to reconcile the different views. Alternatively, you can view it as equal to the gravitational field strength in the frame of reference of an observer on the surface of the earth, which as I mentioned earlier, is a rotating, non-inertial frame of reference.

And, as a side note related to that observation, remember that although centripetal acceleration and gravity are different in cause, a central result of Newtonian physics is that gravity and acceleration at a point are indistinguishable - in other words an infinitesimally small* observer cannot tell whether he is accelerating in an inertial frame of reference versus moving at constant velocity in an inertial frame of reference in the presence of a gravitational field.

*Only holds for infinitesimally small, since for an observer of finite extent, gravitational attraction will vary detectably with R, whereas actual acceleration will not.

Quote:
Originally Posted by Hiro View Post

What is so freaking hard about this?

Well - I'd really like to ask you the same question. Are we done?
post #133 of 147
I love this site! There is no other forum I frequent that can have so many in-depth discussion about so many different subjects. If you can't learn something here you aren't trying.

"The real haunted empire?  It's the New York Times." ~SockRolid

"There is no rule that says the best phones must have the largest screen." ~RoundaboutNow

Reply

"The real haunted empire?  It's the New York Times." ~SockRolid

"There is no rule that says the best phones must have the largest screen." ~RoundaboutNow

Reply
post #134 of 147
Quote:
Originally Posted by muppetry View Post

So - let me just restate the argument one more time to make sure that we are not just talking about different things. Maybe you can identify precisely where you disagree, if you still do.

The original discussion was about variation in weight.

Maybe we can pause right there. Weight and gravity were originally conflated, a problem. All along I have talked only about gravity and the need to respect the difference between gravity and other gravity like forces. Weight is not gravity, weight is the result of a mass accelerated (pulled even if unmoving) by gravity and other forces (centripetal being one) at the Earth's (Moons, etc. ...) surface. That is also roughly the Apparent Gravity discussion on the Wikipedia gravity page which was cited as a counter argument to my calling out the incorrect weight/gravity use, and my counter-counter post that we were in the midst of a bad misinterpretation of gravity and what that wikipage actually said.

All along I have only held that those extra force components of Apparent Gravity are not Gravity, just a composite/resultant force that is very close to the actual force of gravity. That is why I keep saying as far as gravity goes I don't care about the centripetal accelerations.

And that directly extends to weight, which rolls back around to where this all started -- the mess over pounds as mass, pounds as weight (and my snide eventual comment about pounds as currency) and the various early posters seemingly willing to ignore they were dealing with an overloaded term.

The remainder of your post looks mostly OK, up until you call your g the gravity we use on Earth. Your g is not gravity, it is a version of apparent gravity (the interaction between gravity and ideal spherical centripetal acceleration).

It is those little slips in technical terminology precision that end up perpetuating the issue.

Quote:
Originally Posted by muppetry View Post

And, as a side note related to that observation, remember that although centripetal acceleration and gravity are different in cause, a central result of Newtonian physics is that gravity and acceleration at a point are indistinguishable - in other words an infinitesimally small* observer cannot tell whether he is accelerating in an inertial frame of reference versus moving at constant velocity in an inertial frame of reference in the presence of a gravitational field.

That's D'Alambert's Principle (the first part), the second part is explained by Einstein in describing General Relativity. D'Alambert just makes the math easier, and unfortunately easier to safely miss-mash the math together. Convenient for a problem at a time, but the mis-mash methods are not necessarily good for generality. Better to save the vector combinations for as late in the evaluation as humanly possible. General Relativity relates interpretations between frames of reference, not the math in each one (apparently the explanations were a mess before that, but I'm not that old nor a physics historian who keeps track of who though what when).

As for whether or not the gravity and the acceleration-at-a-point are are indistinguishable, that is a 'it depends' statement. To an observer standing on the ground with human senses, yes indistinguishable; to a gravitometer calibrated for lat/long and GPS altitude, no quite distinguishable*. Also they are very distinguishable when you are trying to do physical simulations. If you combine the equations algebraically as you have done the math gets ever more nasty when also doing body-body interactions. When you keep the components separated, use sensible local origins and use geometric methods the math is much simpler (though there are more discrete operations), it's also easier to program and extend for more generality (especially important when you don't know what the next question for the sim will be). Seeing as I'm very involved in the latter issues, the side of 'it depends' I fall on defaults to separate components rather than combined far less flexible solutions (try to dynamically simulate something as simple as long range rocket assisted sensor platform delivery using your combined apparent gravity equation compared to by-component methods).

Quote:
Originally Posted by muppetry View Post

Well - I'd really like to ask you the same question. Are we done?

We should be, it's just a case of say what we mean, mean what we say. Not something almost and incredibly close to like we mean or say. It's just about maintaining discipline when using terminology and I see great benefit with precise use of terminology.


* The recently reaching lunar orbit gravity mapping mission on the moon will be measuring actual Moon gravity by using differences in the orbital mechanics of two satellites in the same orbit causing varying distances between the spacecraft and measuring those relative distances via femtosecond accurate timing methods. That will allow the team to measure local orbital perturbations via the satellites relatively varying centripetal accelerations and then use the information to isolate the actual gravitation component for a given surface grid location. Again very discriminating despite the individual spacecraft effectively "feeling" a unified gravitational and orbital centripetal acceleration.
.
Reply
.
Reply
post #135 of 147
Quote:
Originally Posted by Hiro View Post

Maybe we can pause right there. All along I have talked only about gravity. Weight is not gravity, weight is the result of a mass accelerated (pulled even if unmoving) by gravity and other forces (centripetal being one) at the Earth's (Moons, etc. ...) surface. That is also roughly the Apparent Gravity discussion on the Wikipedia gravity page.

All along I have only held that those extra force components of Apparent Gravity are not Gravity, just a composite/resultant force that is very close to the actual force of gravity. That is why I keep saying as far as gravity goes I don't care about the centripetal accelerations.

Well I'm glad that we are finally getting there, but I did offer this explanation of the disagreement back in #116, and you rejected it outright. Although there are no extra forces involved. I wish you would stop saying that. Never mind. See below.

Quote:
Originally Posted by Hiro View Post

And that directly extends to weight, which rolls back around to where this all started -- the mess over pounds as mass, pounds as weight (and my snide eventual comment about pounds as currency) and the various early posters seemingly willing to ignore they were dealing with an overloaded term.

I won't disagree that other posters became muddled on this - that is why I joined the discussion - and I liked your currency comment.

Quote:
Originally Posted by Hiro View Post

The remainder of your post looks mostly OK, up until you call your g the gravity we use on Earth. Your g is not gravity, it is a version of apparent gravity (the interaction between gravity and ideal spherical centripetal acceleration).

It is those little slips in technical terminology precision that end up perpetuating the issue.

Thanks, but I disagree on two counts:
  1. Mostly OK? Meaning not completely? Don't make that statement without backing it up with specific criticism.
  2. There is no imprecision in that use of g; even though it has been used both to describe the earth's standard gravity (which itself is an average that takes into account rotation) and the local variable value, I defined my use of it as the latter at the outset, so neither imprecise nor ambiguous. I have never seen it used to describe local field strength, which appears to be your preferred definition, but perhaps you can provide a citation for that use.

Quote:
Originally Posted by Hiro View Post

That's D'Alambert's Principle. It just makes the math easier, and unfortunately easier to safely miss-mash the math together. Convenient for a problem at a time, but the mis-mash methods are not necessarily good for generality. Better to save the vector combinations for as late in the evaluation as humanly possible.

No, it's not. It's Einstein's equivalence principle. D'Alembert's Principle (not D'Alambert's) is just a restatement of dynamic equilibrium in a constrained system, and does not mention the equivalence of acceleration and gravity. I've never had to resort to using it.

Quote:
Originally Posted by Hiro View Post

As for whether or not the gravity and the acceleration-at-a-point are are indistinguishable, that is a 'it depends' statement. To an observer standing on the ground with human senses, yes indistinguishable; to a gravitometer calibrated for lat/long and GPS altitude, no quite distinguishable*. Also they are very distinguishable when you are trying to do physical simulations. If you combine the equations algebraically as you have done the math gets ever more nasty when also doing body-body interactions. When you keep the components separated, use sensible local origins and use geometric methods the math is much simpler (though there are more discrete operations), it's also easier to program and extend for more generality (especially important when you don't know what the next question for the sim will be). Seeing as I'm very involved in the latter issues, the side of 'it depends' I fall on defaults to separate components rather than combined far less flexible solutions (try to dynamically simulate something as simple as long range rocket assisted sensor platform delivery using your combined apparent gravity equation compared to by-component methods).

No, there is no "it depends" about it. That particular equivalence principle is completely universal and has no caveats other than the one I quoted (infinitesimally small observer). I have no idea where you got that from, or what you mean by "they are very distinguishable when you are trying to do physical simulations", or where you are going at all with the rest of that comment for that matter, but I would be curious to know. Either you misunderstood my statement of the principle, your simulations are badly flawed, or I am completely missing your point.

Quote:
Originally Posted by Hiro View Post

We should be, it's just a case of say what we mean, mean what we say. Not something almost and incredibly close to like we mean or say. It's just about maintaining discipline when using terminology and I see great benefit with precise use of terminology.

Here I agree entirely, but would point out that I was very careful to define all terms from the outset, made no assumptions or approximations, and dealt only in idealized physics. If you are going to demand precision, you might want to review your own posts. If our original disagreement is just field strength versus local acceleration / weight, then I think that we have resolved that, leaving just the other details that I addressed above.
post #136 of 147
It has been interesting reading both of your arguments. I think you've actually been arguing over the definition of the word weight. One wants it to refer to the force of attraction between two bodies based on mass. The other wants it to refer to the reading on a scale set on the surface of the earth.
post #137 of 147
Quote:
Originally Posted by chabig View Post

It has been interesting reading both of your arguments. I think you've actually been arguing over the definition of the word weight. One wants it to refer to the force of attraction between two bodies based on mass. The other wants it to refer to the reading on a scale set on the surface of the earth.

Well maybe, but more by implication, since Hiro never explicitly disputed it. However, weight is defined as mg and obviously m is invariant with location. So if g does not include all location-dependent factors the factors that affect weight, then either the definition of weight is incorrect, or it is not what we measure on a scale. I doubt anyone would argue the latter, but I've learned not to be surprised at anything I read here.

It is clear to me that Hiro wants g to be used purely to mean the gravitational field strength of the earth - in which case he is correct - it is position dependent only due to the oblateness of the earth, and independent of rotation. But while that is a perfectly valid quantity, the whole argument arose because the precise definition of weight requires a different value for g - the one that I derived that does take into account rotation. That this is correct is trivially obvious; in addition to the very simple but rigorous derivation that I gave, it is covered in countless basic physics texts and even the entry entitled "Gravity of Earth" on the ever-controversial Wikipedia. It is a little unfortunate that Wikipedia screws up the explanation by introducing a centrifugal force (which is nonetheless valid in the less elegant rotating frame of reference), but I think it is just trying to explain it in lay terms.

Anyway, I think the basic disagreement is pretty much resolved.
post #138 of 147
Quote:
Originally Posted by muppetry View Post

Well I'm glad that we are finally getting there, but I did offer this explanation of the disagreement back in #116, and you rejected it outright. Although there are no extra forces involved. I wish you would stop saying that. Never mind. See below.

As for 116 you are reading what you want to see, not what I said. I dismissed the majority of your post precisely because it was about the mathematical contributions of centripetal force. Not gravity. so no need for me to respond to it in detail at all.


Quote:
Originally Posted by muppetry View Post

Mostly OK? Meaning not completely? Don't make that statement without backing it up with specific criticism.
[*]There is no imprecision in that use of g; even though it has been used both to describe the earth's standard gravity (which itself is an average that takes into account rotation) and the local variable value, I defined my use of it as the latter at the outset, so neither imprecise nor ambiguous. I have never seen it used to describe local field strength, which appears to be your preferred definition, but perhaps you can provide a citation for that use. [/LIST]

I did. Which you apparently don't like because I don't agree with the use of g and calling that gravity in your text. The web is also muddled on the topic so most links are irrelevant, I happen to agree with NIST. And I wasn't grading for partial credit so I'm more than willing to say OK on the math without having examined it to the point I would vouch for it. That's not a cut, that's more like I trust you didn't screw up the basics.


Quote:
Originally Posted by muppetry View Post

No, it's not. It's Einstein's equivalence principle. D'Alembert's Principle (not D'Alambert's) is just a restatement of dynamic equilibrium in a constrained system, and does not mention the equivalence of acceleration and gravity. I've never had to resort to using it.

Well we can agree to disagree how to interpret your statements, because on my reading I do see those, I'm not going to argue that your view is wrong because it isn't, it's just at a different abstraction level that derives from the same first principles which is where D'Alembert comes in at a lower level that Einstein (name spelling error inconsequential unless you are trying to be a puffery about it).


Quote:
Originally Posted by muppetry View Post

No, there is no "it depends" about it. That particular equivalence principle is completely universal and has no caveats other than the one I quoted (infinitesimally small observer). I have no idea where you got that from, or what you mean by "they are very distinguishable when you are trying to do physical simulations", or where you are going at all with the rest of that comment for that matter, but I would be curious to know. Either you misunderstood my statement of the principle, your simulations are badly flawed, or I am completely missing your point.

I think you are really missing the point. The sim is well regarded. Not worth arguing over implementation that works, it does and many folks hate the geometric formulations because it's not what is taught in Physics 101, but they make almost everything soooo much simpler.
.
Reply
.
Reply
post #139 of 147
Quote:
Originally Posted by Hiro View Post

Strange stuff in a language I don't understand, but that certainly isn't physics...

We seem to have regressed again, and are probably on the wrong side of the law of diminishing return. Many might say we were there days ago, but perseverance is a virtue isn't it?

Let's just agree to differ then. I'm still frustrated that you never addressed any of my arguments with anything resembling technical responses, even when goaded to do so, but still had the nerve to accuse me of lack of precision and technical understanding. Your prerogative though, to use whatever form of argument works best for you. One of the good and bad things about the internet forum is that, in the end, there is no way to prevail in the face of single-minded and determined opposition. I failed miserably to convince you that mathematics and physics trump all else, and, not surprisingly, you failed to convince me that we should regard the basic laws of physics as fuzzy and open to our individual interpretation. At least that's how I read it.

With your permission I'd like to declare this horse fully expired, or feel free to take a parting shot and do the honors yourself.

Until next time...
post #140 of 147
Quote:
Originally Posted by muppetry View Post

you failed to convince me that we should regard the basic laws of physics as fuzzy and open to our individual interpretation. At least that's how I read it.

Yes the horse is dead, but only because you have decided that if it isn't said exactly how you want to frame it it becomes "fuzzy and open to our individual interpretation" -- even when I cite the National Institute of Standards and Technology as the source of the interpretation I'm going by-- I guess that puts your last statement in total perspective...

I don't need the internet forums to validate my ego, I'll happily continue to let the research contracts/sponsorships be my validation. If I really was as fuzzy and making individual interpretations as you seem to think, the follow on money would have dried up long ago.
.
Reply
.
Reply
post #141 of 147
Quote:
Originally Posted by Hiro View Post

Yes the horse is dead, but only because you have decided that if it isn't said exactly how you want to frame it it becomes "fuzzy and open to our individual interpretation" -- even when I cite the National Institute of Standards and Technology as the source of the interpretation I'm going by-- I guess that puts your last statement in total perspective...

I don't need the internet forums to validate my ego, I'll happily continue to let the research contracts/sponsorships be my validation. If I really was as fuzzy and making individual interpretations as you seem to think, the follow on money would have dried up long ago.

Well damn it - now you've piqued my curiosity again. You didn't cite NIST - a citation is a specific reference to a published work - all you did was claim you got your interpretation from them. However, if you would care to cite something specific from NIST then I'd be happy to take a look at it and explain why it doesn't support your position.

We really are back to the same thing I complained about earlier - I'm giving detailed mathematical proofs while you are busy dismissing them, without any specific criticism, and instead just throwing around unsubstantiated assertions, misusing physics terms and principles, and now name dropping.

But hey - no worries - as long as you are well funded, who cares if you know what you are talking about.

You want some actual citations?

A fairly comprehensive historical description of measurement of local acceleration due to gravity from the National Bureau of Standards:

J. Res. Nat. Bur. Stand. Sec. C: Eng. Inst., Vol. 72C, No. 1, p. 1

A conference paper that describes quite concisely why this matters:

http://www.space-electronics.com/Lit...nt_of_Mass.PDF

And even though you don't like Wikipedia, at least when it disagrees with you, how about the Wikipedia page on standard gravity that spells out in very plain English in the second paragraph (no need to worry about understanding equations as we've established you don't do math) that you are wrong about the correct meaning of g - it is the local value that takes into account centripetal acceleration due to the rotation of the earth.
post #142 of 147
Quote:
Originally Posted by muppetry View Post

Well damn it - now you've piqued my curiosity again. You didn't cite NIST - a citation is a specific reference to a published work - all you did was claim you got your interpretation from them. However, if you would care to cite something specific from NIST then I'd be happy to take a look at it and explain why it doesn't support your position.

Well I did, but when I lost my edit window apparently I didn't get it in the second time around, it was supposed to be in the last sentence of 145.

Quote:
Originally Posted by muppetry View Post

We really are back to the same thing I complained about earlier - I'm giving detailed mathematical proofs while you are busy dismissing them, without any specific criticism, and instead just throwing around unsubstantiated assertions, misusing physics terms and principles, and now name dropping.

But hey - no worries - as long as you are well funded, who cares if you know what you are talking about.

You want some actual citations?

A fairly comprehensive historical description of measurement of local acceleration due to gravity from the National Bureau of Standards:

J. Res. Nat. Bur. Stand. Sec. C: Eng. Inst., Vol. 72C, No. 1, p. 1

A conference paper that describes quite concisely why this matters:

http://www.space-electronics.com/Lit...nt_of_Mass.PDF

And even though you don't like Wikipedia, at least when it disagrees with you, how about the Wikipedia page on standard gravity that spells out in very plain English in the second paragraph (no need to worry about understanding equations as we've established you don't do math) that you are wrong about the correct meaning of g - it is the local value that takes into account centripetal acceleration due to the rotation of the earth.


And all that only says what I have been saying all along; but you refuse to accept gravity is gravity, and the other forces are other forces. You then trot out lots of additional acceleration issues that add up to apparent gravity and then call that gravity. which it isn't.

The Precise Measurement of Mass reference's principles are the same basic ones used for the lunar mission I mentioned earlier where you use knowledge of the centripetal accelerations to isolate the actual gravity. Boynton even used this quote "This example is for the gravitational attraction only and doesn’t include effect of centrifugal force due to earth’s rotation." He then goes on to use that as a starting point in his scale calibrations for location.

EXACTLY the same concept I have been saying all along. Repeatedly.

What part of your own reference is so non-standard that it is what I (in all my supposed non-standard interpretations by your description) have been saying all along?

The other ref link is broken so I won't be able to show how that agrees with me as well.

And OBTW: cheap shots don't prove your point at all.
.
Reply
.
Reply
post #143 of 147
Quote:
Originally Posted by Hiro View Post

Well I did, but when I lost my edit window apparently I didn't get it in the second time around, it was supposed to be in the last sentence of 145.

No problem. So where is it? You still haven't given it.

Quote:
Originally Posted by Hiro View Post

And all that only says what I have been saying all along; but you refuse to accept gravity is gravity, and the other forces are other forces. You then trot out lots of additional acceleration issues that add up to apparent gravity and then call that gravity. which it isn't.

Good grief - are you really that obtuse? No it doesn't, and there are no other forces. How many times do I have to make that point? I worked purely in an inertial frame of reference in which there is no centrifugal force. There is only the attraction due to gravity, and then the residual part of it that is left to be measured as weight after some has been used to maintain circular motion.

Quote:
Originally Posted by Hiro View Post

The Precise Measurement of Mass reference's principles are the same basic ones used for the lunar mission I mentioned earlier where you use knowledge of the centripetal accelerations to isolate the actual gravity. Boynton even used this quote "This example is for the gravitational attraction only and doesn’t include effect of centrifugal force due to earth’s rotation." He then goes on to use that as a starting point in his scale calibrations for location.

And then he explains why you have to take into account the variation of g with location (including rotation) to calibrate your scales to get a consistent value for mass. Note that carefully - variation with location taking into account all effects, including rotation - the entire discussion, encapsulated in that one observation.

In any case - you just demonstrated, I think, that you understand this concept just fine when you quoted "use knowledge of the centripetal accelerations to isolate the actual gravity". That clearly indicates that you realize that the acceleration due to gravity measured on the surface of any rotating body is modified by that rotation.

So let me ask one last simple question - would you agree that except on the axis of rotation (i.e. near the poles) the acceleration towards the surface of the earth of an object when it is dropped is dependent on the gravitational field strength, latitude, and the rotation of the earth? And that the normal reaction between a mass and the surface of the earth (i.e. its weight) depend on the same quantities?

If we don't disagree about that (despite that fact that you have denied it repeatedly), then is your entire problem just centered on whether the symbol g is used to mean locally measured acceleration due to gravity or local gravitational field strength? You do understand that simple distinction, don't you? And you do recognize that I defined right at the outset what I meant by g, consistent with the definition in most texts (although I will say for completeness that I do have one 1971 text on mechanics that breaks that convention and uses g sub e to mean effective local acceleration due to gravity) and consistent with the original discussion in this thread.

Quote:
Originally Posted by Hiro View Post

EXACTLY the same concept I have been saying all along. Repeatedly.

EXACTLY the opposite of what you have been saying all along. Repeatedly.

Quote:
Originally Posted by Hiro View Post

What part of your own reference is so non-standard that it is what I (in all my supposed non-standard interpretations by your description) have been saying all along?

Er, what? My objections to your "interpretations" arise when you make statements such as that the validity of the equivalence principle is not universal (it is), or that sensitive enough measurements can detect the difference (they can't, by definition), or that it is some kind of subset of d'Alembert's principle (it's not), or when you try to obfuscate (I assume that is your intent) by using terms borrowed from computing such as "abstraction level" that have absolutely no meaning in physics. I'm guessing your field is programming, although I'm still surprised that we have struggled so much to find a common language.

Quote:
Originally Posted by Hiro View Post

The other ref link is broken so I won't be able to show how that agrees with me as well.

Apologies - somehow truncated the URL.

Quote:
Originally Posted by Hiro View Post

And OBTW: cheap shots don't prove your point at all.

Which cheap shots would those be? Referencing Wikipedia again? I notice you refrained from commenting on that. Or the comment about funding? Well then don't try to use the fact that you have funding to try to support a technical argument. It's completely irrelevant. Or the comment about math? You repeatedly dismissed my analyses without even checking the trivially simple derivations for the errors or invalid assumptions that presumably you must believe are there. What else should I conclude from that?

Anyway - I don't think there is much more I can add to this topic, so this time I really am done.
post #144 of 147
Quote:
Originally Posted by muppetry View Post

How many times do I have to make that point? I worked purely in an inertial frame of reference in which there is no centrifugal force.

And we are 180 out now where you are flipping use of the terms centrifugal and centripetal. I no longer have to self-chastize myself -- thanks! And how can you say there is no centripetal force in your calculations when you bring in the angular velocity vector ω? I have no response to that except to scratch my head.

Quote:
Originally Posted by muppetry View Post

you realize that the acceleration due to gravity measured on the surface of any rotating body is modified by that rotation.

Given that statement I highlighted with boldface, there is no way to continue the conversation.

You either don't understand component forces, which you seem to understand just fine in your other explanations so I don't think that's it. Or you are really just blindly dug in that there is no difference between actual the gravitational pull of the involved masses compared to the apparent gravity on the surface of a rotating body which combines the gravitational pull between the masses with the effects of the centripetal accelerations.

Either way, there is absolutely nothing anyone on this Earth or any other celestial body in the Universe could say that reconciles with either of those issues.
.
Reply
.
Reply
post #145 of 147
Quote:
Originally Posted by Hiro View Post

And we are 180 out now where you are flipping use of the terms centrifugal and centripetal. I no longer have to self-chastize myself -- thanks! And how can you say there is no centripetal force in your calculations when you bring in the angular velocity vector ω? I have no response to that except to scratch my head.

Given that statement I highlighted with boldface, there is no way to continue the conversation.

You either don't understand component forces, which you seem to understand just fine in your other explanations so I don't think that's it. Or you are really just blindly dug in that there is no difference between actual the gravitational pull of the involved masses compared to the apparent gravity on the surface of a rotating body which combines the gravitational pull between the masses with the effects of the centripetal accelerations.

Either way, there is absolutely nothing anyone on this Earth or any other celestial body in the Universe could say that reconciles with either of those issues.

Once again, you didn't address even a single point I raised in my last post, and instead posted another raft of random bullshit and strawman arguments. So agreed. We are done.
post #146 of 147
I love this forum!

"The real haunted empire?  It's the New York Times." ~SockRolid

"There is no rule that says the best phones must have the largest screen." ~RoundaboutNow

Reply

"The real haunted empire?  It's the New York Times." ~SockRolid

"There is no rule that says the best phones must have the largest screen." ~RoundaboutNow

Reply
post #147 of 147
Quote:
Originally Posted by muppetry View Post

I don't think there is much more I can add to this topic, so this time I really am done.

You are? Could have fooled me!


Quote:
Originally Posted by muppetry View Post

<whinghing snipped> We are done.

Really now? Finally?

Too bad, I really wanted to see if you would take the next step and claim the forces involved in jumping prove anti-gravity.




@SolipsismX, did your popcorn maker get worn out yet?
.
Reply
.
Reply
New Posts  All Forums:Forum Nav:
  Return Home
  Back to Forum: Future Apple Hardware
AppleInsider › Forums › Mac Hardware › Future Apple Hardware › CES: Corning Gorilla Glass 2 is 0.8mm thick, withstands 121 pounds of pressure