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The Forgotten Mystery of Inertia

americanscientist.org

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Re: The Forgotten Mystery of Inertia

#111

Post by Ari_Rahikkala »

eru wrote: post #87
sopooneo wrote: post #41

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…

And interesting enough, the universe does have a unique privileged inertial reference frame: that which makes the background radiation look isotropic.

Only locally, though. The next galaxy cluster over has a uniquely privileged inertial reference frame, too, but it's in motion (and accelerating!) relative to ours.

Re: The Forgotten Mystery of Inertia

#112

Post by twic »

empath75 wrote: post #94
sopooneo wrote: post #41

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…

would it be possible for a drunk in a void to change his rotation?

Yes. He can use the gyroscopic effect of his head spinning.

Re: The Forgotten Mystery of Inertia

#113

Post by Simon_says »

madaxe_again wrote: post #107
CydeWeys wrote: post #104

Earlier quoted context omitted.

The story is phrased in a way that is delicately subtle, and thus elides the mechanism whereby the drunk can change his rotation. Here's a hint: It's relevant that the person is drunk. Another hint: Newton's third law.

I call it my inbuilt vectoring urea reaction engine.

Since you're the only thing in this hypothetical universe, you better make sure your urea makes it to escape velocity (which shouldn't be hard), otherwise you'll make it rain.

Re: The Forgotten Mystery of Inertia

#114

Post by bzbarsky »

bawana wrote: post #102

that a spinning gyroscope does not topple has not much to do with Mach, Einstein, the universe or any such big idea. I can explain it much more simply. Consider a gyroscope spinning fast. so if all of a sudden you try a push its top northward then that instantaneous force pushes down on the north edge of the disk and up on the south edge.Those specific parts of the disk acquire momentum. However because the disk is s…

> Angular momentum is a man made calculation

So is linear momentum, really.

I agree that you can analyze an actual physical gyroscope just in terms of linear momentum without reference to angular momentum, though the calculations end up harder. But there are things that have angular momentum that doesn't obviously correspond to linear momentum (circularly polarized light and nonzero-spin elementary particles come to mind).

Re: The Forgotten Mystery of Inertia

#115

Post by vasjenetehais »

jprissi wrote: post #110

I don't get how the gyroscope is used in missile guidance. How is the gyroscope mentionned in this article pointing at Kiev? How do you read it on the gyroscope and how is the position 'set' in the device?

Actually, the gyroscope will give your angular rotation. It can be achieved through different technics (MEMS gyro, mecanical gyro...) but the final goal will always be to give you your angular rotation.

You need your angular rotation and the specific forces applied to your body (given by the accelerometers) to be then able to compute your position, speed... in the earth frame called NED (North East Down).

That's the very short version. You also need to know your Lla position to correct your gravity model, the error model of your sensor to correct them... Kalman filtering is always used (in my experience).

Here is a draw that seems accurate https://en.wikipedia.org/wiki/Inertial_navigation_system#Met...

Then if you want to go dirty hands, you will need these keywords : "inertial navigation mechanization". This PDF seems clear at first look (5s check) : http://www.ecsutton.ece.ufl.edu/ens/handouts/inertialnav_v2....

Kalman filtering can be overhelming but the course from D.Alazard is really great (still using it after 3 years in navigation field).

If you want a quick exemple, a north finder algo is very easy to understand (basic earth projection). It is also the first step of initialisation of a navigation system.

Re: The Forgotten Mystery of Inertia

#116

Post by AnimalMuppet »

rkagerer wrote: post #21
mikeash wrote: post #12

Earlier quoted context omitted.

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…

Inertia is not a property intrinsic to an object but depends upon all the mass in the universe. That quote from the article really crystallized Mach's Principle for me. So continuing mikeash's great explanation... basically the rest of the universe is pushing the gyroscope into its stable position?

Don't rely much on Mach's Principle. It's a philosophical statement with exactly zero experimental evidence.

General Relativity would say somewhat the same thing, but in a very different form, with different results. GR would say that it depends on the geometry of the local spacetime, which, yes, does depend on all the mass in the universe, but not in the way that Mach's Principle would lead you to expect.

Note that GR, in contrast with Mach's Principle, does have experimental evidence.

Re: The Forgotten Mystery of Inertia

#117

Post by danbmil99 »

Retra wrote: post #43
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?

a standard gyro fights precession at 90 degrees to the attempted off-axis rotation, and shows way more rotational inertia than can be accounted for by the mass involved.

Re: The Forgotten Mystery of Inertia

#118

Post by randallsquared »

philipov wrote: post #103
randallsquared wrote: post #100

Earlier quoted context omitted.

The most fundamental particle can't rotate, because that would imply that parts of it had different velocities from other parts... but it doesn't have parts at all, by initial definition. So, Democritus' atoms imply that rotation is not a fundamental motion. I think every attempt to visualize this which imagines a single object rotating is doomed. Rotation inherently implies a difference in velocities, and some attra…

Okay, then explain Spin. Angular momentum is defined by how a particle responds to fields, not by the relation of its parts.

I'm no physicist, of course. I had thought that 'spin' was metaphorical or analogous, like quarks' color and flavor, but Wikipedia asserts that it was originally considered to be rotation about an axis. So, I've no real clue. :)

Re: The Forgotten Mystery of Inertia

#119

Post by goialoq »

snowwrestler wrote: post #9
goialoq wrote: post #6

> If it is spinning extremely rapidly, the gyroscope remains rigidly locked in the direction it has been set, its sights fixed on...Kiev—hence the term inertial guidance systems. This is clearly false... right? If you translate the gyroscope, it won't be pointing at Kiev anymore, it will be pointing to the side of Kiev. The gyroscope doesn't magically point at a target, it provides a stable reference direction that t…

He elides the inner workings of an inertial guidance system because it's not really necessary. The point is that the gyroscope provides an absolute reference point, which seems weird in the face of relativity.

I agree, but the intro is all about how this magical gyroscope is pointing at Kiev, even making a point about how the gyro is doing it after being separated from the rest of the rocket.

Re: The Forgotten Mystery of Inertia

#120

Post by bawana »

mannykannot wrote: post #105
bawana wrote: post #98

Earlier quoted context omitted.

a spinning disk 'averages' its mass distribution. Consider a gyroscope spinning fast. so if all of a sudden you try a push its top northward then that instantaneous force pushes down on the north edge of the disk and up on the south edge.Those specific parts of the disk acquire momentum. However because the disk is spinning, the part of the disk that was north soon becomes south. and the southern piece that was movin…

Yes, though I think you can avoid the question of why just "those specific parts of the disk" by considering a dumbbell (two joined, point masses spinning around an axis perpendicular to the center of the rod joining them) rather than a disk or ring, and a continuous torque. If you are pushing northwards on the axis, the maximum rate of change of the instantaneous velocities of the point masses is when they are align…

indeed it tips perpendicular to the direction of applied force. and this is what happens in reality. try spinning a bicycle wheel holding its axle on both ends. now try to twist the axle and it curiously wants to move the wheel in a perpendicular direction.
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