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[Iacopo]

Did you push the wheel in the direction of the precession?

Yes, I pushed the wheel in the direction of the precession.  If I don't, the wooden axis in any case stays horizontal, but the wheel tilts down to about 45 degrees.  Instead if I push it, I can start the wheel with its axis parallel to the wooden axis, which is more interesting.  Whatever I do, apart from oscillations, the wooden axis stays always horizontal.

[Jeremy McCreary]

Great experiment, Iacopo! Thanks for doing it!

Your result is what I would have expected and I am still scratching my head about Laithwaite's demo.

My sentiments exactly.

Did you push the wheel in the direction of the precession?

I definitely saw a push in the precession direction in Iacopo's test. Didn't see one in Laithwaite's case but might have missed it.

[Iacopo]

I definitely saw a push in the precession direction in Iacopo's test. Didn't see one in Laithwaite's case but might have missed it.

This anyway doesn't explain the difference in the results we had.
I couldn't give enough speed to the wheel, so also I pushed it to help it not to tilt down too early.

[Jeremy McCreary]

His one hour Christmas lecture on gyroscopes can be found in 19 parts at this site link = gyroscope lecture.... It is a lot of show with no explanation

John,

Do you have any thoughts on the demonstration Laithwaite gave at 6:04 in segment 4/7 below?



It has several of us stumped.

[Russpin]

Do you have any thoughts on the demonstration Laithwaite gave at 6:04 in segment 4/7 below?

What I think is happening is this:
when the gyro is hanging straight down and balanced with the weight on the right side of the beam, the center of mass of the system is below the beam pivot and is stable. With gyro horizontal the center of mass of the system is above the pivot and is unstable. If you watch the video closely by pausing and starting you can see he releases the weight on the right first and then releases the gyro causing a tilt with the weight lower. Because the center of mass of system is above the beam pivot and unstable the weight continues to go down. 

[ta0]

Do you have any thoughts on the demonstration Laithwaite gave at 6:04 in segment 4/7 below?

What I think is happening is this:
when the gyro is hanging straight down and balanced with the weight on the right side of the beam, the center of mass of the system is below the beam pivot and is stable. With gyro horizontal the center of mass of the system is above the pivot and is unstable. If you watch the video closely by pausing and starting you can see he releases the weight on the right first and then releases the gyro causing a tilt with the weight lower. Because the center of mass of system is above the beam pivot and unstable the weight continues to go down.

I think you are into something, Russpin.
I imagine the pivot (fulcrum) of the bar is close to its center so it is actually close to stable for any angle of inclination. On the other hand, in Iacopo's experiment the fulcrum is above the center of the beam, what would make it stable when the beam is horizontal like in a regular beam balance.

Johnm has several educational gyroscopes in his lab. He could certainly do this experiment . . .

[Russpin]

when the gyro is hanging straight down and balanced with the weight on the right side of the beam, the center of mass of the system is below the beam pivot and is stable.With gyro horizontal the center of mass of the system is above the pivot and is unstable.

I just realized that the pivot between the gyro and beam makes the above statement not valid. The system with this pivot can not be treated like a rigid body.

However I still find the tilt the gyro makes with the beam at the end of video strange, like he may have given a push to the beam at release.

[Iacopo]

I imagine the pivot (fulcrum) of the bar is close to its center so it is actually close to stable for any angle of inclination. On the other hand, in Iacopo's experiment the fulcrum is above the center of the beam, what would make it stable when the beam is horizontal like in a regular beam balance.

This I see as a part of the answer but still I think there is something else:
the fulcrum nearer to the center means a weaker force to keep the bar horizontal, but anyway it still should stay horizontal.
I think that if I build the same gyroscope as we see it and I test it, in my case the bar would stay horizontal.
It seems to me that there is some inaccuracy in the construction of Laithwaite's gyroscope:
Look at the very first seconds here:

A possibility is that the axis of the hanging gyro is not free to reach 90 degrees inclination from the bar; if the center of mass of the gyro with its axis can't stay exactly under the pivot with the bar, when he balances the bar, this will affect the balance.  Even only 2 or 3 degrees of the gyro axis tilted outwards could be enough: this would have the same effect as the gyro was suspended to the bar, not where there is the pivot, but a bit more far from the center of the bar.  It would be like there is a bit more weight in this arm of the bar, (the arm a bit longer).
This would be compensated with the counteweight in the other arm of the bar a bit more far from the center of the bar.
When the gyro spins, its weight will be on the pivot, which is closer to the center of the bar, so it will appear to be lighter.

I'm not sure that this is the explanation, but it seems possible to me. 

[Russpin]

A possibility is that the axis of the hanging gyro is not free to reach 90 degrees inclination from the bar; if the center of mass of the gyro with its axis can't stay exactly under the pivot with the bar, when he balances the bar, this will affect the balance.

Good observation. I think this may be the best explanation so far. Of course it's all a bit of speculation without actually examining the equipment in person.

[ta0]

A possibility is that the axis of the hanging gyro is not free to reach 90 degrees inclination from the bar; if the center of mass of the gyro with its axis can't stay exactly under the pivot with the bar, when he balances the bar, this will affect the balance.

Good observation. I think this may be the best explanation so far. Of course it's all a bit of speculation without actually examining the equipment in person.

I agree, that would explain the behavior of the gyro in a simple way. In fact, when he pushes down on the other arm it is pretty obvious that the gyro tilts up, demonstrating that it reached a stop and it is not free hanging (anymore?). However, it won't explain how Laithwaite did not realize this as it should have been pretty obvious to him that the pivot did not let the gyro hang freely. 

[johnm]

We don’t have a gyroscope with a jointed arm like that one in our collection.   This design is much too complicated to be a useful teaching aid.

Since I stopped thinking long ago, I asked a physics professor who is sympathetic to my addiction (OK, sympathetic is strong, but he at least doesn’t laugh anymore when he sees my stacks of plastic buckets), but admits his mechanics is weak.  His first guess based on my chalkboard sketches without watching the video, is that for a well-designed gyroscope balance for which the unbalanced state is not too far from the balanced state (which the demonstrated one may not be), the joint in the arm should allow two stable configurations while the gyroscope is “alive”, one with the horizontal gyro above and one with the horizontal gyro below, each achieved as the “effective” lever arms (horizontal component upon which gravity acts) and mass combinations of the two sides match (m₁l₁=m₂l₂) as the main beam tilts at the primary fulcrum and the joint makes a corresponding bend keeping the gyro horizontal.  To which equilibrium position the system moves depends on the initial conditions of release, basically if he starts slightly above the horizontal (or some critical angle)  it goes up, and if he starts below the horizontal (or some critical angle) it goes down.  The demonstrated system is perhaps not well designed to clearly show the two states since it tilts an extreme amount at the primary fulcrum such that the beam contacts the support post before the equilibrium position is reached which demonstrates well the effect he is promoting.

However, it won't explain how Laithwaite did not realize this as it should have been pretty obvious to him that the pivot did not let the gyro hang freely. 

People often only see what they want to see or get stuck on their first interpretation.  It can happen to the best of us   
http://www.ta0.com/forum/index.php/topic,2632.msg26950.html#msg26950

[the Earl of Whirl]

I am glad I am not the only one johnm has caught.  He says he has stopped thinking but his memory sure is strong!!!  Go johnm!

[Iacopo]

two stable configurations while the gyroscope is “alive”, one with the horizontal gyro above and one with the horizontal gyro below

Since I have not studied physics, I easily may be wrong, anyway still I don't see reasons for the beam not staying always horizontal. Maybe one day I will have some fun building this kind of gyroscope, and if I make it, I will let you know how it behaves, (some kind of gyroscope I know I will want to make in any case, they are fascinating objects).

[ta0]

I am glad I am not the only one johnm has caught.  He says he has stopped thinking but his memory sure is strong!!!  Go johnm!

John never forgets!  But I have to confess that that one is worth remembering! 

I believe that conventional physics (as opposed to Laithwaite's version) tells us that the gyro in a state of constant precession will apply all its weight at the pivot point, regardless of its inclination. That is equivalent to hanging a weight from its pivot or to just adding a point mass to the arm at that location. So it can be treated just like a rigid arm free to rotate around the main pivot. Let's assume the right and left sides of this virtual arm are statically balanced around the main pivot. If this pivot or fulcrum is at the height of the center of mass, any angle of inclination will be semi-stable. If the pivot is higher than center of mass, the stable position will be with the center of mass directly below it and that would make the arm horizontal. If the center of mass is above the pivot point, the balance is unstable and the arm can fall to either side (lowering the center of mass).

On the video, the arm appears to pivot around a pin at the center of the arm, what would make any angle stable if Laithwaite correctly balanced it with the hanging non-spinning gyro. But when he lets go the gyro it is not yet precessing and it will initially tilt and fall. So for a fraction of a second it will apply less weight to the arm, until precession takes over. So you expect the pivot to bend the way it does. If this would be enough to explain how the gyro ends up, I am not sure 

[Iacopo]

On the video, the arm appears to pivot around a pin at the center of the arm, what would make any angle stable if Laithwaite correctly balanced it with the hanging non-spinning gyro. But when he lets go the gyro it is not yet precessing and it will initially tilt and fall. So for a fraction of a second it will apply less weight to the arm, until precession takes over. So you expect the pivot to bend the way it does. If this would be enough to explain how the gyro ends up, I am not sure 

I didn't consider the case of the pivot at the exact center of the beam, also if, (I watched the video again), it can be easily seen that this is probably the case of Laithwaite's gyro. 
I have to say that the your seems to me a very good explanation.
I too had the same effect, releasing the spinning wheel; if I don't push it to help precession, the wheel tilts down and at the same time the arm raises up a bit.  Then in my case the arm oscillates up and down because it wants to stay horizontal, but if the pivot was at the center of the beam, it would not come back, and the gyro would stay up.

Also, now I understand the explanation given by Johnm, which is what it happens with the pivot slightly below the center of the beam.