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The Forgotten Mystery of Inertia

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Re: The Forgotten Mystery of Inertia

#41

Post by sopooneo »

A drunk awakens in a void and finds his arms, legs, in fact all extremities are bing pulled outward by some reverse gravity. It is quite unpleasant.

By some strange mechanism he is able to make himself start spinning. At first it is worse: in one direction the reverse gravity increases in effect! So he reverses and is relieved to find that in the other it slowly abates. Though the sweet relief is strange. Because if he goes too far in that direction, he finds that the expulsatory force begins to increase again.

But eventually he finds the sweet spot and stays there, relieved, to find himself not being pulled apart. "This", he says to himself, "I shall call an inertial reference frame. It is so much more relaxing than the others."

Re: The Forgotten Mystery of Inertia

#42

Post by Neeek »

justinpombrio wrote: post #17
xenadu02 wrote: post #15

Earlier quoted context omitted.

In physics (and in many other areas) it is unsatisfying to wave your hands and say "it is so because our measurements prove it". A theoretical model that explains why the measurements come out the way they do is far more valuable because it leads to other insights. We can predict the behavior of a gyro but there is still a fundamental question of what a gyro is relative to since GR tells us there is no universal refe…

> Are gyroscopes entirely local or does the gravity of distant galaxies present a sort of universal reference frame? Or is it something else entirely? Something else entirely. You don't need other galaxies or a universal reference frame. Even if the gyroscope were the only thing in the universe, it would be easy to tell if it were spinning: a very tiny person standing on the inside rim would be held down if it were s…

which is obvious because a gyro is not rigid and has parts the move relative to each other, but would the entire system feel a force if exerted on it and correct it's orientation? What would it be "pointed" at? If it did, wouldn't that imply an absolute point of reference?

Re: The Forgotten Mystery of Inertia

#43

Post by Retra »

danbmil99 wrote: post #39

One question I've never seen answered to my satisfaction: If you created a gyroscope with two counter-rotating disks somehow identical in moment and location, would it fight precession symmetrically, or act like a block of matter with nothing rotating inside it?

Aren't those the same alternative? If something is fighting precession symmetrically, then isn't it acting like a block of matter with nothing rotating inside it?

Re: The Forgotten Mystery of Inertia

#44

Post by westoncb »

lutorm wrote: post #29
westoncb wrote: post #16

Earlier quoted context omitted.

Hmm, but if the perihelion changes position and the orbit doesn't change shape, the only other option is that the entire orbit shifts—is that it? My understanding of precession is that there are two levels of rotation: an object rotating on some axis A, with axis A rotating around another axis B. So in the case of the perihelion precessing, what would be the corresponding parts? I was assuming the only precession tak…

"Precession" is just another word for rotation. It sounds like you're thinking of a spinning top, whose rotation axis will precess (rotate) around another axis (usually the local vertical). In the context use here, the precession in question is the rotation of the perihelion (or, if you want to think of it more akin to the above case, the semimajor axis of the ellipse) that rotates in the orbital plane (around the no…

Thanks, that article does clear it up. So the orbit doesn't change shape nor does it shift, instead it rotates (from the article: "More precisely, it is the gradual rotation of the line joining the apsides of an orbit"—so I guess the ellipse major axis rotates).

From what I can tell using precession as a direct substitute for rotation is less common, even in astronomy—but maybe I've just had poor samples so far. I was thinking of it as: "Precession is a change in the orientation of the rotational axis of a rotating body." (https://en.wikipedia.org/wiki/Precession)

Re: The Forgotten Mystery of Inertia

#45

Post by AgentME »

kirrent wrote: post #38
AgentME wrote: post #31

Earlier quoted context omitted.

>Certainly if you were alone in the universe, he thought, there would be no way to tell if you were rotating, so there could be no centrifugal force if you started to spin with respect to, say, another person. Isn't that... intuitively wrong though? If you were spinning fast enough in space, your body would be ripped apart. If you were spinning and let go of an object near you, instead of floating in place, it would…

I'd agree with your intuition in both cases, but it's worth pointing out the equivalence principle (falling in an elevator) isn't necessarily immediately obvious (which I know is not something you claimed). Feynman complained about philosophers who would claim it's obvious that motion was always relative but then wouldn't understand that the same thing didn't apply to rotational motion. http://www.feynmanlectures.cal…

Let me refine that part: I think that what would physically happen in the elevator example is apparent to most (someone with knowledge of Newtonian physics may agree with the idea that the interior of a falling elevator acts identically with the interior of an unmoving gravity-less elevator), but people without knowledge of relativity and the experiments that lead to it may disagree about what the elevator example means about physics and the world.

But the spinning-in-an-empty-universe example is a situation that I don't even agree with Mach/Einstein about what would happen, much less about what it says about the world. Maybe they're right, but this difference makes the thought experiment a much different kind and arguably a less compelling one than the other.

Thanks for the link, I found it an interesting read!

Re: The Forgotten Mystery of Inertia

#46

Post by phonon »

danbmil99 wrote: post #39

One question I've never seen answered to my satisfaction: If you created a gyroscope with two counter-rotating disks somehow identical in moment and location, would it fight precession symmetrically, or act like a block of matter with nothing rotating inside it?

https://www.youtube.com/watch?v=vGun5athdfg

Re: The Forgotten Mystery of Inertia

#47

Post by evanb »

naasking wrote: post #26
dingo_bat wrote: post #10

Maybe I'm wrong but it's wrong to say we don't know how a gyro works. We can accurately and precisely predict the behavior of a gyro, derive the mathematical equations necessary and the model we use is general for all moving objects. This is the greatest level of knowing you can have. Maybe the author meant that a gyro's motion is not intuitive to most humans. That I can agree with. For me, even the fact that my smar…

> This is the greatest level of knowing you can have. If it doesn't answer "why?", then it's incomplete. Perhaps at some point in our explorations of physics there will be axioms we must simply accept, but the gyro isn't it. We should have a reason for its behaviour.

Gyros aren't the axiom, they're a theorem---the axioms are Newton's laws of motion.

Re: The Forgotten Mystery of Inertia

#48

Post by thaumasiotes »

apsophus wrote: post #36
letlambda wrote: post #24

Earlier quoted context omitted.

Walk around to the opposite side of the gyro wheel. If the acceleration is due to the rotation of the gyroscope, it still points out radially. If it was due to a mass out beyond the rim, you're falling on your head.

What if the mass is distributed in a ring surrounding the gyro at a distance? How would you distinguish between spinning versus mass in that case? General Relativity may seem counterintuitive, but experimentally it really does seem to be the way our universe works. Starting out from the position that absolute motion is meaningless has proven remarkably fruitful. It's also something a lot of very smart people have bee…

> What if the mass is distributed in a ring surrounding the gyro at a distance? How would you distinguish between spinning versus mass in that case?

In that case, it wouldn't be drawing you outward no matter where you were located. So, spinning would be "you feel a gravitational force", and a ring of mass would be "you don't feel a gravitational force".

Re: The Forgotten Mystery of Inertia

#49

Post by epistasis »

Neeek wrote: post #42
justinpombrio wrote: post #17

Earlier quoted context omitted.

> Are gyroscopes entirely local or does the gravity of distant galaxies present a sort of universal reference frame? Or is it something else entirely? Something else entirely. You don't need other galaxies or a universal reference frame. Even if the gyroscope were the only thing in the universe, it would be easy to tell if it were spinning: a very tiny person standing on the inside rim would be held down if it were s…

which is obvious because a gyro is not rigid and has parts the move relative to each other, but would the entire system feel a force if exerted on it and correct it's orientation? What would it be "pointed" at? If it did, wouldn't that imply an absolute point of reference?

Not an absolute point of reference, but an inertial point of reference:

https://en.wikipedia.org/wiki/Inertial_frame_of_reference

Re: The Forgotten Mystery of Inertia

#50

Post by egocentric »

mikeash wrote: post #12
westoncb wrote: post #7

Anyone know of (or want to give) a clear explanation of how gyros behave? I found the article to be unclear on a number of points. I was also unclear on this: "The first triumph of general relativity was its exact prediction of the orbital precession of Mercury’s perihelion" —does a perihelion precess? I thought it was the planet which was precessing and the perihelion shifted (changing the shape of the planet's orbi…

Gyros are really simple! Think of the gyro's ring as a series of independent particles flying around really fast in a horizontal circle. Now you push up at one point on that circle. Does the circle move up at that point? No. You're applying a force to each particle only very briefly before it moves past and you're pushing on the next one. But you do impart some upward velocity on the particle during its brief time un…

I once built a simple mass-spring physics engine, and this exact effect what was causing the wheels of my car-like mass-spring blob to bend "funny". It was only after I slowly debugged it (by removing friction, then gravity, softer springs, slower timescale etc) that I felt I really got how these forces come about.
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