The Forgotten Mystery of Inertia • Hacker News Board

Re: The Forgotten Mystery of Inertia

#91

Post by phkahler » Tue, Oct 17, 2017, 1:35 PM UTC

monochromatic wrote: post #40

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…

This is exactly what Mach’s principle disputes.

Then Mach's principle is a straw man, we do not have access to an empty universe.

Re: The Forgotten Mystery of Inertia

#92

Post by phkahler » Tue, Oct 17, 2017, 1:41 PM UTC

xenadu02 wrote: post #15

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…

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…

>> 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 reference frame.

If one can see that the gyro is still pointing in the same direction, then one has already defined a reference frame. I can see confusion if we had a gyro spinning inside a free-falling elevator (or even a closed box spinning in space). You can still tell the gyro is spinning because there are internal stresses that indicate the axis of rotation, if not the direction. The earth bulges at the equator because of its rotation, it would require much force to keep it spherical.

Re: The Forgotten Mystery of Inertia

#93

Post by mikeash » Tue, Oct 17, 2017, 1:43 PM UTC

mannykannot wrote: post #81

mikeash wrote: post #79

Earlier quoted context omitted.

You can imagine each particle being somehow tethered to the center, without any need for the particles to be connected otherwise. You’re totally right that gyroscopes are straightforward if you understand angular velocity as a vector quantity, but it’s not quite so intuitive.

If each particle is only tethered to the center, how does the circle bring it down again? If the tethers are all fixed to one another, we're back to rigidity. Edit: I'm trying to think of a way to bring in angular momentum without having to derive it in full generality...

You could imagine that they’re connected to the center of rotation with a string or something. You just need centripetal force, not a rigid connection to anything. Although that may still be more than what you’re after....

Re: The Forgotten Mystery of Inertia

#94

Post by empath75 » Tue, Oct 17, 2017, 1:51 PM UTC

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?

Re: The Forgotten Mystery of Inertia

#95

Post by e12e » Tue, Oct 17, 2017, 1:59 PM UTC

> Would you feel centrifugal forces in an empty universe? Does the law of inertia mean anything in an empty universe? Mach would give a resounding “no” to both questions: Inertia is not a property intrinsic to an object but depends upon all the mass in the universe.

If "you" can exist in a massless universe (that which encompass all that exist) - then, by definition, "you" have no mass. So insofar as mass is connected to inertia is connected to angular momentum (the need for acceleration/force to alter the direction of movement) - then: no, you would not be able to detect "centrifugal forces" without any mass.

But, if "you" have mass and exist in a universe, that universe consist of at least the mass making up "you". Let's assume a sphere/clump of small magnets. Would it be possible to spin this sphere fast enough for it to disperse - tear itself apart? I would think so.

Re: The Forgotten Mystery of Inertia

#96

Post by philipov » Tue, Oct 17, 2017, 2:00 PM UTC

lmm wrote: post #74

Ended wrote: post #64

Earlier quoted context omitted.

>Rotation is inherently about different particles moving at different velocities. This is why there is an absolute reference frame for it: it is defined by these differences, so by reducing them to zero and making every particle in the ball have the same velocity, we can reach the "absolute". Imagine the ball is floating in space, and you are watching things through a camera fixed to the ball. To you, the two dots wi…

But if you were the camera you'd feel a force, or not, right? It's not like being in an elevator in free-fall, where you can't tell whether you're accelerating downwards or sitting still in flat space.

The elevator analogy breaks down if you're larger than a point-particle in a point-elevator. Gravity's force varies with distance, causing tidal forces on your body, allowing something large/sensitive enough to feel the difference. Gravity stretches you whereas uniform acceleration does not, and non-uniform acceleration compresses you.

In other words, you can determine that you're in a gravitational field by measuring the difference in force at different locations in the elevator.

Re: The Forgotten Mystery of Inertia

#97

Post by dmikalova » Tue, Oct 17, 2017, 2:02 PM UTC

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?

You could blow in one direction or the other, at an angle. The air you push away will push you the other way.

Re: The Forgotten Mystery of Inertia

#98

Post by bawana » Tue, Oct 17, 2017, 2:04 PM UTC

mannykannot wrote: post #81

mikeash wrote: post #79

Earlier quoted context omitted.

You can imagine each particle being somehow tethered to the center, without any need for the particles to be connected otherwise. You’re totally right that gyroscopes are straightforward if you understand angular velocity as a vector quantity, but it’s not quite so intuitive.

If each particle is only tethered to the center, how does the circle bring it down again? If the tethers are all fixed to one another, we're back to rigidity. Edit: I'm trying to think of a way to bring in angular momentum without having to derive it in full generality...

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 moving up is now north.now the momenta of the influenced disk segments act to right the gyroscope.

The same effect can occur during the acceleration of a gyroscope in a missile in space. Even tho there is no gravity. The effect of acceleration on the gyroscope is the same, causing it to maintain its orientation in space.

The faster a gyroscope spins, the more quickly it rights itself. and thus appears more stable. as it slows down, the righting effect fails.

Re: The Forgotten Mystery of Inertia

#99

Post by philipov » Tue, Oct 17, 2017, 2:20 PM UTC

dmikalova wrote: post #97

empath75 wrote: post #94

Earlier quoted context omitted.

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

You could blow in one direction or the other, at an angle. The air you push away will push you the other way.

If you're suspended in a fluid or gas, you're not in a void.

Re: The Forgotten Mystery of Inertia

#100

Post by randallsquared » Tue, Oct 17, 2017, 2:29 PM UTC

librexpr wrote: post #63

PeachPlum wrote: post #60

Earlier quoted context omitted.

Is the particle in the centre moving at all? Are the particles point mass?

I don't think a point can rotate. What would it mean if it did? There would be no observable change. So no, I don't think the absolute center point moves. From a physical standpoint, though, if you wanted to talk about the absolute center point, you'd have to talk about the lowest level of particles, which gets into the weirdness of quantum mechanics (or maybe a lower level exists?). At that point, I really couldn't…

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 attractive force keeping those velocities changing to maintain a distance. So, the existence of any attractive force, together with inertia, implies rotation and the stickiness of direction of a gyroscope. But inertia itself already implied stickiness of direction, given that a change in direction would require acceleration...