“Why does gyroscope levitate?” stumps 3 physics profs

[CWatters]

The key to understanding why gyroscopes don't levitate is that they don't produce any linear forces - only torques. If you try to make them into a levitating machine the gyro simply tries to bend the machine rather than lift it.

 

[rcgldr]

no net torque on the gyroscope ...

I corrected my previous post. A net torque on a gyroscope results in precession coexistant with an internal torque that opposes the net torque but doesn't change the fact that there is still a net torque exerted onto the gyroscope.

 

[A.T.]

an internal torque that opposes the net torque

The net torque is the sum of all external torques and fully determines how the angular momentum changes. So whatever that " internal torque" is supposed to be, it's not relevant here.

 

[VantagePoint72]

@rcgldr, I always appreciate it when someone acknowledges having made a mistake, but I think it would be best if you just did so in a post of its own rather than going back and editing all your past posts. This whole conversation is now very confusing to anyone just joining because you've removed all the incorrect statements from your previous posts. This is not good forum practice.

 

[cj]

This is very helpful - thank you.

From nearly the beginning we said we realized we might not be asking a valid question. We asked, in an early post, for someone to answer the question as asked, or politely suggest how the question itself might be better phrased. Aside: we did indeed also post about a possible analogy to the gravitational/centripetal metaphor you mention.

It is disconcerting to have moderators not read the entire post(s). Also, simply posting links to sources isn't exactly a value-added service (especially since we clearly said we visited all the usual suspect sites). Do moderators actually get paid for this kind of service? Lastly, getting hung up on arcane definitions ("levitate") that are not germane to the real issue is off-putting, as is terse/sarcastic/rude responses. This is true for anyone, and it's especially true for moderators.

Lastly, for the umpteenth time we jokingly used the term "...stumped the internet." If you can't sense this, then either get out and socialize more, and simply ignore it. I mean, really?

Sorry for the rant, but a few of us in my group are very cognizant of polite customer service. I think PF needs a jolt of it.

I think this analogy is very useful, and I want to expand it a little. Farina and cj, terminological issues aside, I think the issue here isn't necessarily that you're not understanding the explanations you're being given. It's that you're not seeing why your original question isn't actually well-formed. There's a very particular way of explaining the problem that you want to be given. The problem is that such an explanation does not exist because it's not the reason for the phenomenon in question. I'm going to use voko's analogy to clarify what I mean.

I gather from your question that what you want is an explanation in terms kind of some force that acts directly on the free end of gyroscope and holds it up, i.e. an upward torque to counter the downward torque of gravity. There is no such counter-torque, and to insist on being shown one is to misunderstand what is going on during gyroscopic precession. Consider a planet orbiting the sun. Gravity tends to pull it inwards, and yet a circular orbit allows it to stay a fixed distance from the sun. Now imagine you came to the forum and asked:

"Why does an orbiting planet stay a fixed distance from the sun? What is the force that keeps it from falling inward? In the case of no angular velocity, the planet simply falls towards the sun. In the orbiting case, there is no radial acceleration so some force must be opposing gravity to keep it from falling. Three physics professors and the entire internet have not been able to give me an explanation in terms of force equilibrium of how orbiting planets don't fall toward the sun."

Do you see how this question is based on a fundamental misunderstanding? Of course there isn't a force opposing gravity, that's the whole point—there's net centripetal acceleration. Forces are not balanced along the radial axis! Sure, if you really wanted to, you could think in terms of the planet's non-inertial reference frame and conclude the the centrifugal force exactly opposes the gravitational force according to the planet. But that is a lot of unnecessary work for what is not really a good, fundamental explanation based on inertial reference frames. The question has wrong assumptions built right into it about how orbits work, and no amount of correct explanations would convince the asker until they were able to see that.

The same is the case here, with really the only difference being that the vectorial nature of torques and angular momentum is harder to picture than the vectorial nature of linear forces and momentum. There is no torque acting on the other side of the gyroscope to keep it from flopping over. We would not see precession if there were! The torque about a horizontal axis due to gravity is manifestly not balanced by another torque because precession occurs, just as the inward force on an orbiting planet is manifestly not balanced some outward force because circular orbital motion occurs!

Spend some time reflecting on this and hopefully you will see that you did not stump the internet; you only stumped yourselves.

 

[rcgldr]

@rcgldrhaving made a mistake, but I think it would be best if you just did so in a post of its own rather than going back and editing all your past posts.

Well the intent was to avoid mis-leading anyone reading those posts with bad information (those mistakes) before continuing on only to find out that those posts were in mistake.

I editted the prevoius post to note the original post had mistakes before the correction and that later quotes were made of the mistaken original post.

What I was thinking of was the effect on the frame that supports the gyroscope. To that frame, the gyroscope is yet another external source of torque and/or forces, and while the gyroscope is in precession and experiences a net torque, the frame that supports the gyroscope does not experience a net torque, because the gyroscope exerts a countering torque onto the frame, but those are torques exerted onto the frame, not onto the gyroscope.

 

[A.T.]

Sorry for the rant, but a few of us in my group are very cognizant of polite customer service. I think PF needs a jolt of it.

Ask for your money back.

 

[russ_watters]

It is disconcerting to have moderators not read the entire post(s). Also, simply posting links to sources isn't exactly a value-added service (especially since we clearly said we visited all the usual suspect sites).

Mea culpa. I should have stayed out. rcgldr's post was good and posted at the same time as mine (I didn't see it until after I posted). It didn't seem to me that you guys got that precession was the phenomena you were viewing, but for whatever reason I didn't say that -- I might have been short on time.

Do moderators actually get paid for this kind of service?

No, we are all voluteers.

Lastly, getting hung up on arcane definitions ("levitate") that are not germane to the real issue is off-putting, as is terse/sarcastic/rude responses. This is true for anyone, and it's especially true for moderators.

Lastly, for the umpteenth time we jokingly used the term "...stumped the internet." If you can't sense this, then either get out and socialize more, and simply ignore it. I mean, really?

Sorry for the rant, but a few of us in my group are very cognizant of polite customer service. I think PF needs a jolt of it.

Those are on Farina. Definitions matter quite a lot in science, and attitude matters in conversation. We're all human here: When people detect an attitude from a poster, they sometimes respond back with it. So starting the thread off better would have led to better results -- and this threat was at risk of being locked without an adequate answer because of it. We never approach a thread/new poster intending to be mean, but at the same time this isn't a restaruant where we have to take abuse from "customers" no matter what. That's something you guys are going to need to learn: it may be unpleasant to hear that you are wrong sometimes, but in the real world, people are going to tell you bluntly and if you refuse to accept it, treat you roughly.

 

[Bandersnatch]

Well the intent was to avoid mis-leading anyone reading those posts with bad information (those mistakes) before continuing on only to find out that those posts were in mistake.

I find it a good practice in such cases to use the s and /s tags to mark the error without deleting the context.

 

[VantagePoint72]

This is very helpful - thank you.

You're welcome.

Lastly, getting hung up on arcane definitions ("levitate") that are not germane to the real issue is off-putting, as is terse/sarcastic/rude responses.

The levitation argument was entirely on Farina. She misused a term, was informed so, and rather than accepting the correction she dug her heels in. She continued to insist on using non-standard, confusing terminology and started giving an enormous amount of attitude. Perhaps some other posters took it too far in response, but the tone of this thread was, in the first, place set by Farina.

 

[cj]

I think many of us take a different view. You might not realize this but a very sizable number (at least most of our fellow students here) shy away from PF. The consensus here seems to say getting a helpful response is not worth the snarky-ness one has to endure (we have social media for that ;)). For me, I'll take a right answer and pay for it via absorbing less-than-cordial responses. I might be, though, in the minority. A lot of people seem to go to Khan, MIT, etc. There seems to be a happier vibe there. I think Farina was obstinate, and the moderator was less than professional, i.e. condescending/combative. Again, this is the polite customer service gene acting up in me.

At any rate, thank you again.

You're welcome.
The levitation argument was entirely on Farina. She misused a term, was informed so, and rather than accepting the correction she dug her heels in. She continued to insist on using non-standard, confusing terminology and started giving an enormous amount of attitude. Perhaps some other posters took it too far in response, but the tone of this thread was, in the first, place set by Farina.

 

[Dale]

Sounds like the scientific question is answered and now we are just arguing over style.

I personally will take your feedback into consideration in the future, although I am not sure what the best approach is. Possibly I should have simply deleted the OP.