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The Forgotten Mystery of Inertia

americanscientist.org

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Re: The Forgotten Mystery of Inertia

#21

Post by rkagerer »

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…

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?

Re: The Forgotten Mystery of Inertia

#22

Post by QAPereo »

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…

This is a description of rotational stability, gyroscopic stability is: http://www.real-world-physics-problems.com/gyroscope-physics...

Key point

Gyroscopic stability also explains why a spinning axisymmetric projectile, such as a football, can have its symmetric (long) axis stay aligned with its flight trajectory, without tumbling end over end when in flight. The spin imparts a gyroscopic response to the aerodynamic forces acting on the projectile, which results in the projectile long axis aligning itself with the flight trajectory. The physics involved here is a combination of gyroscopic analysis and aerodynamic force analysis due to drag and (potentially) the Magnus effect. This is quite complicated and will not be discussed here. However, there is a lot of literature available online on gyroscope physics, as related to projectile spin and gyroscopic stability, if one wishes to study this topic further.

Re: The Forgotten Mystery of Inertia

#24

Post by letlambda »

fiddlerwoaroof wrote: post #19
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…

But that doesn't distinguish between the gyroscope spinning and the existence of a sufficiently massive object in the appropriate place, right?

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.

Re: The Forgotten Mystery of Inertia

#25

Post by lutorm »

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…

Sure. It also has to be powered up and aligned to a known attitude and position within a not-too-distant past, because actual gyroscopes have friction and are not perfectly locked in space. Either the guy didn't actually know how they worked or he was pulling a prank.

Re: The Forgotten Mystery of Inertia

#26

Post by naasking »

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.

Re: The Forgotten Mystery of Inertia

#27

Post by headShrinker »

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…

I'm not a physics major. So I may have made a major booboo in explaining this.

I get what you're saying. We know what inertia does. We have the math to estimate is effect. We can measure it's affect. To be clear, we don't know how it works. In the vacuum of space, if your body begins to spin, a force will pull your arms away from your from your body. No one on this planet can unequivocally explain why this happens. No one can explain the mechanism for how gravity works. No one can explain why when something starts spinning, there is a force that makes it continue to spin, or why there is a need for force to stop it.

These are basic forces and our lack of understanding shows just how little we understand about the universe. This, after the smartest people, over 2,000 years have had a crack at it.

Steve Jobs said that people who have made everything are no smarter than you or I. Einstein said something similar. I find it a bit reassuring that some of the worlds most basic forces are yet unexplained. We are still in our infancy of figuring things out. Everyone should get to work. There's a lot to do.

Re: The Forgotten Mystery of Inertia

#28

Post by colordrops »

QAPereo wrote: post #20
colordrops wrote: post #18

So there was the animation that Caltech put out today or yesterday of the neutron star collision that was just detected, and it visualized gravitational waves moving outward from the stars: https://www.youtube.com/watch?v=e7LcmWiclOs It just reinforces the (incorrect?) intuition that spacetime is "something" rather than nothingness between things. Why can't motion be relative to spacetime rather than other objects?

I think you’ve confused the notion of nothingness, which may or may not even be real in practice, and spacetime, which is definitely a thing. Gravity is a result distortions in that thing.

Ok, so if that's the case, why can't objects be relative to spacetime rather than other objects, as discussed in the article?

Re: The Forgotten Mystery of Inertia

#29

Post by lutorm »

westoncb wrote: post #16
thatcherc wrote: post #13

Earlier quoted context omitted.

My (undergraduate) understanding of the precession of the orbit of Mercury is that the longitude of the periapsis in Sun coordinates changes over time. The shape of the orbit stays the same, unless maybe it's changed a bit by perturbations from the outer planets. Classically, you would expect the shape and position/rotation of the orbit to be constant. In reality, its rotation in the orbital plane changes due to GR.

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 normal vector to the orbital plane).

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

Re: The Forgotten Mystery of Inertia

#30

Post by lutorm »

thatcherc wrote: post #13
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…

My (undergraduate) understanding of the precession of the orbit of Mercury is that the longitude of the periapsis in Sun coordinates changes over time. The shape of the orbit stays the same, unless maybe it's changed a bit by perturbations from the outer planets. Classically, you would expect the shape and position/rotation of the orbit to be constant. In reality, its rotation in the orbital plane changes due to GR.

its rotation in the orbital plane changes due to GR.

Actually, it would change even without GR. The orbital ellipse is only constant in the ideal two-body problem. The fact that planets aren't perfectly spherically symmetric, and the presence of the other planets, also cause the orbits to precess.

From the Wikipedia article I linked in my other post: "For Mercury, the perihelion precession rate due to general relativistic effects is 43″ per century. By comparison, the precession due to perturbations from the other planets in the Solar System is 532″ per century, whereas the oblateness of the Sun (quadrupole moment) causes a negligible contribution of 0.025″ per century."

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