Why is it so much easier to lift a spinning gyro than if stationary?

  • #51
jbriggs444
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That does not realy answer the question and sounds cryptic.What was required was an unamabigiouse reply to the question of where exactly is the centre of mass of a spinninig gyro on a stand or string.The implication is that the CoM it is at the centre of the spinning mass but that mass is precessing about a point,"the stand".The mass moves around a point and the answer seems that the point becomes the centre of pressure rather than the CoM.
Definitions define experiments dictate.
The Center of Mass is a defined position. It is where it is defined to be. Experiment cannot change this. An experiment that measures something else... measures something else.
 
  • #52
Simon Bridge
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This was my own idea I confess I was in error !
... that is a good quality in a scientist.
It is good to think about things and formulate theories - it is better to present them as ones own theories until you have verified they are true.

The usefulness of a spinning disk in a navigation device is that it tends to stay aligned to the ether/remote galaxies/CMBR or what ever the latest theory is I confess I did not appreciate the purpose gimbals [served] that allow the body of the device to move without disturbing the alignment of the spinning wheel.
Please read the links I provided you in my replies ... you have been laboring under a common misunderstanding about how gyroscopic navigation works. Although - certainly - the phenomenon of interest is that a gyro tries to maintain whatever orientation it was in when it was spin up: the gyro does not care about CMBR or what distant galaxies are doing.
 
  • #53
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The original question was long ago answered by the video. I think it was not clear that the problem is more one of beam loading than gyroscopes. The spinning mass at the end of the shaft produces an end moment when the professor rotates it about his body. The support condition changes from two hands providing a moment to support an overhanging load to a single hand providing the forty pounds to raise the weight and an end moment to counter the moment of the mass at the end of the shaft. Because of the conservation of angular momentum a moment exists at the weighted end of the shaft. The professor then only needs to provide the forty pounds to lift the weight. There is no change in weight but the device is easier to lift.
 
  • #54
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Hi all, oh well, it seems the thread grew some limbs...:)
I don´t know, it is most certainly unnecessary but, If it is of any need at all I just wanted to say that I got my answer during the the very first few posts. It would a long story to explain how . But in short, the whole problem was related to my mechanical replication of the experiment with wrist, elbow and shoulder articulation of the gyro. I made a mistake in construction one of the joints and that screwed my otherwise healthy interpretation of the gyro.
 
  • #55
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The Center of Mass is a defined position. It is where it is defined to be. Experiment cannot change this. An experiment that measures something else... measures something else.
I agree except that the defined position might not be where it expected to be.There don't seem to be a consensus that the CoM is changeing in it's position as the giro revolves about the stand.This is not a fixed position!You can define the Centre of Mass of a revolving can full off marbles before it rotates but once it moves you cannot.
 
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  • #56
jbriggs444
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I agree except that the defined position might not be where it expected to be.There don't seem to be a consensus that the CoM is changeing in it's position as the giro revolves about the stand.This is not a fixed position!You can define the Centre of Mass of a revolving can full off marbles before it rotates but once it moves you cannot.
Expectations are irrelevant. The center of mass is where it is defined to be. That position is determined by the definition, although it may vary over time. It will not, in general, be directly above the support point. The center of mass of a revolving can full of marbles is perfectly well defined.
 
  • #57
A.T.
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.There don't seem to be a consensus that the CoM is changeing in it's position as the giro revolves about the stand.
The CoM is static relative to the gyro. It doesn't move closer to the support.

You can define the Centre of Mass of a revolving can full off marbles before it rotates but once it moves you cannot.
Utter nonsense.
 
  • #58
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The original display (which I saw) was on the BBC Tomorrow's World programme; the stationery gryoscope stayed down whilst a small girl swung on the other end of a see-saw. With the gyroscope spinning the girl could lift the gyroscope with one finger pressing down on the other end of the see-saw. The newspapers later reported that Laithwaite was sent to Coventory by Cambridge University but, refused to retire. When he did retire due to age (65), all his papers were destroyed.
Clearly the weight of the gyroscope does not change but, as every baryon (i.e. proton) has a shell of two gravitons, it is the position of the weak gravitons that is altered by spin, creating a local gravity field around the mass of the gyroscope.
 
  • #59
PeterDonis
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as every baryon (i.e. proton) has a shell of two gravitons
Huh? Where are you getting this from?

it is the position of the weak gravitons that is altered by spin, creating a local gravity field around the mass of the gyroscope.
No, this is not correct. The gravitational field of the gyroscope itself is much, much too weak to affect any of the observations under discussion.
 
  • #60
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Expectations are irrelevant. The center of mass is where it is defined to be. That position is determined by the definition, although it may vary over time. It will not, in general, be directly above the support point. The center of mass of a revolving can full of marbles is perfectly well defined.
Maybe a revolving can full of marbles is not the best example of a spinning object in which the centre of mass is not well defined.So here is one that is http://video.mit.edu/watch/double-pendulum-6392/ If it were possible to define the centre of mass before the Chaotic Pendulum was spun, then it should be also possible to determine the centre of mass, the exact position and speed of the pendulem at any given time when it is spinning.As this is clearly not the case then the centre of mass must change position as the object rotates.
 
  • #61
jbriggs444
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Perhaps we are speaking past each other. The word "define" does not mean the same thing as "calculate", "measure", "predict" or even "determine". I use it in the sense of a mathematical definition. To define the center of mass is to explain what the words "center of mass" mean.

The center of mass of a Chaotic Pendulum is perfectly well defined at all times. By that I mean that we know the meaning of the words "center of mass of a Chaotic Pendulum" at all times and further, that there is no ambiguity -- there is, in principle, exactly one position that is the "center of mass".

What is missing in the case of a Chaotic Pendulum is method for making a precise prediction of that center of mass, given a set of measurements, no matter how precise, of the initial conditions. It is, of course, possible to determine both the position and speed of the center of mass of the pendulum at any given time while it is spinning. It's just not possible to predict this position very far in advance.
 
  • #62
A.T.
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Maybe a revolving can full of marbles is not the best example of a spinning object
And neither is your chaotic pendulum, as there is nothing chaotic about a mounted gyro. And as jbriggs noted, you are confusing defining and predicting. And on top of that, your example is contradicting your own claim, that the CoM moves over the support point when the gyro precesses, because that would be very predictable and not chaotic all.



 
  • #63
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Perhaps we are speaking past each other. The word "define" does not mean the same thing as "calculate", "measure", "predict" or even "determine". I use it in the sense of a mathematical definition. To define the center of mass is to explain what the words "center of mass" mean.

The center of mass of a Chaotic Pendulum is perfectly well defined at all times. By that I mean that we know the meaning of the words "center of mass of a Chaotic Pendulum" at all times and further, that there is no ambiguity -- there is, in principle, exactly one position that is the "center of mass".

What is missing in the case of a Chaotic Pendulum is method for making a precise prediction of that center of mass, given a set of measurements, no matter how precise, of the initial conditions. It is, of course, possible to determine both the position and speed of the center of mass of the pendulum at any given time while it is spinning. It's just not possible to predict this position very far in advance.
I don't recognise that we are speaking past each other it's just that there is a probably a misunderstanding between the mathematical definition you propose and the experimental evidence that I try to explain.Mathematicaly it's obviouse that the CoM should be where it's calculated to be within a chaotic system but expermentaly it is obviouse it's not.Otherwise you would have a pendulum moveing in completly predicatable way.
 
  • #64
jbriggs444
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This is not a disagreement about physics. This is a disagreement about linguistics -- about the meaning of words. I did not propose a mathematical definition. I tried to explain what a "definition" (mathematical or otherwise) is.

The CoM of any system is always where it is defined to be. That is true by definition. No possible experiment can demonstrate otherwise. The fact that the CoM of a chaotic system cannot be predicted reliably far into the future is completely irrelevant. You persist in confusing "define" with "predict".
 
  • #65
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The fact that the CoM of a chaotic system cannot be predicted reliably far into the future is completely irrelevant.
Especially since Buckleymanor proposes that the gyro's CoM changes in a very predictable way, as soon as the precession starts. Nothing chaotic about that.
 
  • #66
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This is not a disagreement about physics. This is a disagreement about linguistics -- about the meaning of words. I did not propose a mathematical definition. I tried to explain what a "definition" (mathematical or otherwise) is.

The CoM of any system is always where it is defined to be. That is true by definition. No possible experiment can demonstrate otherwise. The fact that the CoM of a chaotic system cannot be predicted reliably far into the future is completely irrelevant. You persist in confusing "define" with "predict".
Well I can't realy argue past that as you recently posted this,." I use it in the sense of a mathematical definition"You have to make your mind up.
 
  • #67
A.T.
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You have to make your mind up.
Seems to me like almost everyone is pretty clear here, including the thread starter, who already acknowledged that his question was answered on the first page.
 
  • #68
PeterDonis
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Thread closed for moderation.
 

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