I agree. It’s probably pretty unethical to use. You never know whether the person is already carrying an antidote to it. You’ll probably only get the truth as the person believes it, not objective truth. I bet it’s not even admissible in wizarding court.
Yes and no. It helped me grasp something about how the wheel being a gyroscope aids in keeping the bike upright and why a counter-rotating wheel cancel that effect out.
I am a physics teacher. Which probably explains why I got something out of that video and you didn’t. Defining the torque as a vector going out the axle is, as Dr.Strangelove pointed out not nonsense. It’s essential to being able work out the consequences of torque. Physics has a lot of these vectors that don’t seem to make sense unless you start using them. They’re called pseudovectors and the sign of the magnitude is a convention, but people who do a lot of physics tend to forget to mention that.
Using vectors is essential to doing any physics that’s not just linear. If you want to understand physics try to move past that anger and look into some other uses of pseudovectors. And if you do, try to imagine describing the angular momentum and the effect of torque on the angular momentum in some other, more intuitive way, and still use single vectors. It’ll be messy, I promise you.
They “handwave” because the details are very hard. As I said, I am a physics teacher, and I still haven’t found an explanation that shows why gyroscopes behave the way they do that I can grasp intuitively.* And you’ll note that none of the posters that rag on the Veritasium video have offered a “Feynmanian” way of explaining this so a highschooler can understand what is fundamentally happening.
*Admittedly I haven’t searched very hard. Thankfully I’ve never had to teach a syllabus including torque as a vector. It was removed at some point to make room for the much more understandable special relativity.
I am not good enough to keep a motionless bicycle up (but track racers are, see “trackstand”), but if I am going even very slowly, barely moving and too slow for a significant gyro effect, it is pretty easy to stay up. I may be unconsciously providing steering input to keep it stable; I would be surprised if the two videos of stable riderless bikes would work at very slow speeds.
Yeah, I think what the OP is looking for is an intuitive explanation for precession, which would then give an intuitive explanation for why two oppositely-spinning wheels on the same axis would have no precession.
I have this memory from long ago that as a high school kid, I wondered how gyroscopes do their thing, and was looking forward to learning that in college. But then after I got out of college with an engineering degree, I remembered that thought, but realized I still didn’t know. I had gone through the math of it in physics classes, but didn’t have that intuitive grasp of it.
The closest I got was (I think) here on the SDMB many years ago, when someone offered an explanation involving a spinning tetherball and introducing a force in one direction when the tetherball passed by you - the result of the force is that the plane of spinning of the ball tilts, but in a direction 90 degrees away from the place where I applied the (momentary) force.
Somebody tried on Wikipedia, but it’s long and rambling:
No need to guess what Feynman might have said about gyroscopes. You can refer to what he did say:
Does this explanation work for you @Cardinal? And does it help you understand the issues you had with the anti-gyroscopic bikes?
Agree that an intuitive understanding of precession is … difficult.
But I don’t see how that is any obstacle to understanding how two counter-rotating wheels cancel precession. e.g.
Proof:
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A wheel of mass and mass distribution X moving at rotational speed Y exerts precessive force (torque) Z about the axle because … magic.
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An identical wheel of the same mass and mass distribution X moving at the same rotational speed Y in the opposite direction exerts precessive force (torque) Z’ about the axle in the opposite direction because … magic.
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The force Z and the equal but opposite force Z’ cancel each other.
QED. With no need to understand anything about the “magic”.
I can picture a student thinking about the question in the OP this way though:
- A rotating wheel resists changing the direction of the axis.
- This is why a rotating wheel helps keep the bike upright.
- Another rotating wheel would just add more resistance.
That’s what I’m seeing in the OP.
This is basically what I meant about the additional rotating wheels. It turns out that the gyroscopic effect in general is much more complicated that I was ever told. I’m willing now to accept that the precession of oppositely spinning wheels might cancel each other out. The entire explanation of precession seems to defy many people’s ability to explain without pretending that there are forces and momenta where there clearly aren’t any really.
Feynman’s explanation is a good starting point if you accept conservation of angular momentum as a prerequisite. Sitting in and swivel chair, an turning a spinning wheel from a horizontal axis to vertical must result in additional off-axis forces, because at the end you must be rotating in the chair (to counter the angular momentum of the wheel).
But if you had two wheels spinning oppositely, and turned together, then at the end you will not be swiveling in your chair (no total angular momentum to counter). Therefore the additional off-axis forces aren’t needed.
Try this video. It talks about motion related to spinning. At the 6 min mark is a nice explanation of gyroscopic precession.
My 2 cents:
Gyroscopic effects are minimal for bicycles, I would guess. I saw a similar demonstration in first year physics as in the first video - but it appears the video, like the demonstration I saw, has a heavily weighted rim and I would be curious what speed the wheel seems to be representing - for a 3-foot diameter wheel, the rim circumference is almost 10 feet or so, and 60mph/100kmh would be about 9revolutions/sec. Plus, none of these demonstrations address what the effect is of two spinning wheels in bicycle formation rigidly attached.
We ride a bicycle (unless you’re a circus performer with an excellent balance skill) by going forward. As you start to fall, you lean, you turn the wheel slightly to compensate so centrifugal force from the turn itself pushes you upright… and then start to fall the other way, turn again, back and forth in very slight adjustments. Basically, you steer into the direction you are falling. Most obvious at very slow speed.
Where the gyroscope effect is most noticeable is driving a motorcycle. You have a much heavier front wheel turning at higher speeds. The effect is most noticeable if you want to turn - to turn left, for example, you slightly push/pull the handlebars right, the opposite direction; this creates the gyroscopic action to make the wheel - and bike - lean the direction(left) that then takes you into the leaning turn and you can turn the handlebars as needed.
This effect I noticed most obviously because decades ago I tried riding one of those trike ATV’s after riding a lot of motorcycles. I found myself drifting over the wrong direction (toward the ditch! Yikes!) instead of steering away from the ditch. The problem is that a trike is counter-intuitive to an experienced bike rider because it does not lean. You simply turn the handlebars where you want to go. Your subconscious learning is dangerous. No wonder these things were banned.
Well, several things are happening in the bike wheel demonstrations that are different from an actual bike. The pivot point is at the center of rotation and not in the plane of rotation. For these reasons the demonstration needs more angular momentum to succeed.
An actual bike wheel doesn’t need that much angular momentum to stay upright. The effect can be seen at very slow speeds. That’s partially because the pivot point is far away from the center of rotation and inline with the plane. And also due to friction with the ground.
I’ve seen the Spinning video before, in the past couple days as I’ve tried to explain this all. I’m still convinced these people are skipping steps, and I don’t know if it’s because they assume their logic is inherently apparent, or because they’re not practiced in explaining things thoroughly. I’ve seen both. I’ve learned to explain things in slightly different ways and to anticipate questions and objections and give multiple examples.
At 11:00, I just don’t get what he means. Gravity tries to pull the top of the wheel down to our right, and if it’s spinning, the pieces at the top take time to move away from the string, so the defection passes mostly to 90 degrees downstream from the top, to the point that is away from the camera. Instead of the top moving to our right, the part toward the wall moves to our right.
Right?
What he skips over is how that deflection keeps changing. I know that gravity keeps pulling, but he doesn’t mention how this can keep going. The parts of the wheel keep passing through the different points of the rotation and have very little time to affect the turn. It might sound obvious how this works, to you, but I can’t make it work, and he never mentions it.
I can explain an AP Chemistry test through with almost no prompting sight-unseen, but these guys can’t get precession across to me. I blame them. I know I’ve made some things seem too easy to some people when explaining things, but I wasn’t making a video with the time to write a script and edit. So either they’re skipping points or the topic is too complicated to make a no-math explanation, or it involves completely unintuitive things like virtual photons, and it’s pointless, or they’re skipping points in the explanations.
This being in just text is not good enough. I realize your job is not to make animations for message board questions, but I just don’t know what he means.
" Angular momentum around a vertical axis cannot change because of the (frictionless) pivot of the chair". No idea what the frictionless pivot of the chair is. None.
" But the system (wheel, ourself, and chair) cannot have a vertical component" As far as I can tell, this is not explained.
It’s the swiveling bearing of a swivel chair. The friction is negligible at short timescales.
Since the pivoting axis is upright, it can’t transmit angular momentum on that axis. And there is no other way to get rid of angular momentum without getting rid of mass or touching something. So there has to be an explanation for why the total angular momentum is zero but the wheel is still spinning with a vertical axis.
I doubt that. Chemistry at that level is virtually all empirical rules, with no explanation for why those rules are true. You have to get pretty deep into quantum mechanics before you even get the beginning of a satisfying explanation for chemistry.
Gimme till the weekend. Busy right now.
No, I can explain an AP Chemistry test to a student without having seen the questions before, to the satisfaction of a student who actually cares and studies. I can assimilate and retain information. That’s my point.
All right, say I accept that H+H => H2+energy. You tell me that the 1s shells accept two electrons each and that is why H2 is stable. So why do electron shells exist at all, and why does the 1s shell accept exactly two electrons?