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

[jethomas3182]

a sideways force that is getting transformed into an upward force

"... violates conservation of momentum"

Oops. What's the right way to say that?

It creates a torque that tends to rotate the gyroscope at right angles to the direction of motion, that is, up. You don't notice the opposing torque on you as a weight on your arms.

Something like that?

 

[curiouschris]

No one here has answered the question. which makes me even more curious as to why it is easier to pick the weight up over his head.

Lets get a couple of things straight. there is obviously no cheating in the video's.

Regardless of how you hold the weight. it weighs 40Kg. The use of a lever amplifies the torque on the wrist many fold so while he can pick the weight up over his head when it is not spinning he can only do so by holding it as close to the weight as possible. If he attempted to do it from the end of the axle (lever) it is extremely doubtful he could do that.

When it is spinning he lifts it rather easily.

Now clearly he is lifting it in the direction it is precessing. My guess is he could not force it to go in the other direction and if he attempted that it would drive itself into the ground.

But the question remains. why is it easier to lift in the direction of precession and presumably harder the other way(yes that's an assumption).

Yes the spinning weight has much more potential energy due to its spin but how is that transferred in such a way as to make the lifting significantly easier?

Shouldn't it still weigh the same, and therefore require the same effort?

The answer stating the duration of time it takes to lift the weight is quite clearly wrong. this is not a block and tackle where the weight is supported by other means.

 

[jethomas3182]

Curiouschris, it is because the gyroscope is spinning.

Why does that change anything? He's holding the end of a long lever. If he tries to torque up or down, the gyroscope will move sideways. If he tries to torque sideways, the gyroscope will tilt up or down.

He tries to spin it around his head, and it resists that but it smoothly goes up. He isn't pushing it up, he's only holding it, but it goes up. He doesn't think about the sideways force he's applying because it's easy to do, and it isn't pushing up.

When he brings it down, first the scale sees the weight go down and then later it goes up. That happened only a little bit at a time when he was raising it.I think he thought it was easy because he didn't have to push it hard upward to make it go up. He didn't notice how hard he was pushing it sideways.

 

[hmmmok]

because as the mass of gravity approaches zero the mass in the centrifugal force equation increases to its max.

 

[Simon Bridge]

No one here has answered the question.

Yeah they have - A.T. provided a decent description in an earlier post.

... which makes me even more curious as to why it is easier to pick the weight up over his head.

The devil is in the details: he didn't "pick the weight up over his head", he gave it a shove to the side and then got out of it's way.
... it can take a bit to wrap you head around it though: Where did the description, in A.T.s post, lose you?

People who are still confused may need a bit of a primer:
http://hyperphysics.phy-astr.gsu.edu/hbase/gyr.html

... the key to the "effortless lift" effect is to understand about gyroscopic precession:

From there the discussion about how the precession leads to the ease (or otherwise) of lifting should be easier to understand.

Note:
The motion of the demonstrator should not be separated from the motion of the gyro.
Notice how none of the demonstrators tried to lift while turning the gryo against it's precession?
We cannot discount the human "impression" effect - esp. "effort" is subjective and poorly defined.

The interested student should analyse the forces and torques in the following situations:
1. Does holding the gyro in place require less effort while it is spinning than when it is not?
2. If the spinning gyro was not held totally in place, just holding it up, what would happen?
3. Is lifting the spinning gyro above the head without turning it (i.e. as a dead lift) easier than when you turn it as you lift it?
... note: these are guiding questions to help students.

 

[syhprum1]

I think a lot of the misunderstanding about "gyros" is due to the friction in the bearings of simple improvised ones a proper gyro with quasi zero friction bearings does not precess

 

[curiouschris]

Thanks Simon.

I actually missed the explanation video A.T. posted. Not paying enough attention. I just thought it was a repeat of an earlier video.

I noted in that second video that when he tried to force the gyro against its direction of precession (not sure what its really called) he couldn't do it. which confirms my intuition about it.

 

[Simon Bridge]

I think a lot of the misunderstanding about "gyros" is due to the friction in the bearings of simple improvised ones a proper gyro with quasi zero friction bearings does not precess

Do you have a reference for that? You seem to be saying that, in the absence of friction in the bearings, the spinning gyro will just fall over from the gravitational torque.

I noted in that second video that when he tried to force the gyro against its direction of precession (not sure what its really called) he couldn't do it. which confirms my intuition about it.

... that intuition being:

Now clearly he is lifting it in the direction it is precessing. My guess is he could not force it to go in the other direction and if he attempted that it would drive itself into the ground.

... then yes, that is correct.
Your other questions in the same post are answered in the videos.

 

[syhprum1]

Do you have a reference for that? You seem to be saying that, in the absence of friction in the bearings, the spinning gyro will just fall over from the gravitational torque.

... that intuition being:... then yes, that is correct.
Your other questions in the same post are answered in the videos.

 

[syhprum1]

Visualise the usual toy gyroscope end on with the wheel spinning clockwise looking at the bearing the friction between the rotating shaft and the bottom of the bearing will tend to push the shaft to right hence the precession.
In professional gyros as are used for navigation either air or magnetic bearings are used to reduce this friction as much as possible as precession is definitely not wanted

 

[A.T.]

proper gyro with quasi zero friction bearings does not precess

Precession is caused by a torque perpendicular to the axis. Frictional torque in the bearings, which is parallel to the axis, is not required for precession.

http://en.wikipedia.org/wiki/Gyroscope#Properties
"It follows from this that a torque τ applied perpendicular to the axis of rotation, and therefore perpendicular to L, results in a rotation about an axis perpendicular to both τ and L. This motion is called precession. "

 

[A.T.]

because as the mass of gravity approaches zero

What is "the mass of gravity"?

the mass in the centrifugal force equation increases to its max.

Why should any mass increase here? The mass of the gyro is constant.

 

[Buckleymanor]

Yes, a definition is always correct, per definition.

That does not really 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.

 

[Simon Bridge]

Visualise the usual toy gyroscope end on with the wheel spinning clockwise looking at the bearing the friction between the rotating shaft and the bottom of the bearing will tend to push the shaft to right hence the precession.
In professional gyros as are used for navigation either air or magnetic bearings are used to reduce this friction as much as possible as precession is definitely not wanted

OK. Then this will be amply documented - please provide a reference to the documentation. Thanks.

I am somewhat familiar with professional gyrocompasses - but the one's I know about will not work if the gryo inside them does not precess.
See: http://en.wikipedia.org/wiki/Gyrocompass
http://books.google.co.nz/books?id=YlEEAAAAMBAJ&pg=PA82&redir_esc=y#v=onepage&q&f=false
... please provide a reference to a gyro navigation system where precession is a disadvantage.

You can use friction to provide the torque to get a gyrocompass to precess properly, but that is not the only way - it is usually more practical to use weight, like in the videos. Air or magnetic bearings are used to reduce friction so the gyro stays spinning for a long time - not to stop it precessing.

Toy gyros also roll in a circle due to the contact not being a point. This can look a lot like precession.

... see below for the standard models for gyro precession:
http://hyperphysics.phy-astr.gsu.edu/hbase/gyr.html
http://www2.eng.cam.ac.uk/~hemh/gyroscopes/onetofour.html
... notice that the precession appears as a result of fundamental conservation laws, even in the ideal case of zero friction in the bearings?

But perhaps I am misunderstanding you?
Where did you get the idea that gyroscopes will not precess without friction?
Please provide an example of a gyroscopic navigation where precession is undesirable.
... these will help me to understand you.

 

[syhprum1]

This was my own idea I confess I was in error !

 

[A.T.]

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.

No, that is not the answer. It is trivial to design a stand where the CoM precesses around a different point, than the center of pressure (support point).

 

[syhprum1]

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 severed that allow the body of the device to move without disturbing the alignment of the spinning wheel.

 

[curiouschris]

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 severed that allow the body of the device to move without disturbing the alignment of the spinning wheel.

From what I understand it simply stays aligned to itself and the universe be damned.

 

[A.T.]

it simply stays aligned to itself

That's a tautology. It's true for every rigid body in any situation.

 

[sophiecentaur]

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 severed that allow the body of the device to move without disturbing the alignment of the spinning wheel.

No. The gimbals are there to stop the oven tipping over and spilling your stew onto the floor when the ship heels over. (Engineering priorities; eating beats navigation)

 

[jbriggs444]

That does not really 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.

 

[Simon Bridge]

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.

 

[Mesafarmer]

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.

 

[leviterande]

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.

 

[Buckleymanor]

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.

 

[jbriggs444]

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.

 

[A.T.]

.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.

 

[jhmar]

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.

 

[PeterDonis]

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.

 

[Buckleymanor]

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.