OT: Ceres more interesting than previously thought

[narf poit chez BOOM]

It would take more energy for it to spin slower, and more mass for it to spin faster.

...Did I miss some basic physics here? Last time I checked, heavier things need more energy to move, and more energy makes things go faster.

[inigma]

narf poit chez BOOM said:

...Did I miss some basic physics here? Last time I checked, heavier things need more energy to move, and more energy makes things go faster.

Mass and energy convert between one or the other. The thing with gravity is, you need mass not energy.

Since I believe the spin of a body in space is naturally dependent on mass, by adding mass the spin would increase. By adding energy (which cancels out mass) spin would decrease.

[Suicide Junkie]

inigma said:

Mass and energy convert between one or the other. The thing with gravity is, you need mass not energy.

Since I believe the spin of a body in space is naturally dependent on mass, by adding mass the spin would increase. By adding energy (which cancels out mass) spin would decrease.

Heck no.
E = MC^2, remember? Increasing energy -> increasing mass (by a very small amount since c^2 is huge)

The angular momentum of a body depends on its shape, mass and spin rate.
If you make the object smaller it spins faster since the angular momentum must be conserved.
If you add mass without changing the angular momentum, the body will be spinning slower because there is only so much momentum to go around.

The spin of an asteroid is essentially random as the tiny rocks whang it from random directions at random speeds. However there is an upper limit to how fast they could be spinning, since they are generally not solid bodies.
If they spin too fast then rocks on the surface will be swung off by the centrifugal force, and the asteroid breaks up.

[inigma]

If they spin too fast then rocks on the surface will be swung off by the centrifugal force, and the asteroid breaks up.

That's my point. To generage 1G of centripetal, the astroid would fly apart, or be so big that you wouldn't need to hollow out the core.

E=MC^2 does not take into account gravitational physics. Einstien couldn't figure out exactly how it played into it.

It is my belief that mass contributes to the natural spin of a body in space-time, up to a certain equilibrium point.

Even though you are correct that initially adding mass to Ceres would cause it to slow down, equilibrium would be reached over time and it would naturally speed up by gravitational physics up to a certain point of equilibrium.

Adding too much mass would obviously cause Ceres to spin down too quickly and the weakened gravity well would essentially force Ceres apart.

What I'm essentially saying is that the faster mass moves, the greater the gravity effect - up to a certain point of course where centripetal force starts to overcome object's structural integrity. This is all due to the increase in mass as energy is converted to mass to move something faster.

By forcing Ceres to spin faster, that balance is offset, and eventually cetripetal force will fly it apart. Mathematically, for purposes of this coin idea, I can guestimate that the centripetal force of a spinning body can never be greater than its optimal gravitational force (and structural integrity).

In short, by the time you could spin Ceres to 1G centripetal force, its would fly apart as 1G of centripetal force would be greater than the escape velocity of the surface which is dependent on the equilibrium of 0.03Gs (guessing its 0.03Gs) generated by Ceres.

That 0.03G is what is holding Ceres together. Force Ceres to spin at a 1G centripetal rate, and you can imagine the concequence.

[Suicide Junkie]

What the heck are you talking about? Where did 1G come in?

And WTF is "natural spin"? There is absolutely no preferred spin direction around here in the universe.

Adding too much mass would obviously cause Ceres to spin down too quickly and the weakened gravity well would essentially force Ceres apart.
No way.
More mass -> more gravity.
Slower spin -> less centrifugal force.
Which means objects on the surface feel even more force downwards.
The rate of mass change is completely irrelevant.

This is grade nine or lower physics here.
Please, look up ANGULAR MOMENTUM.

[inigma]

My info on "natural spin" comes from Dr. Ning Li formerly of the University of Alabama.

I had meant to say that "adding too much mass too quickly would obviously cause Ceres to spin down and the weakened gravity well would essentially allow Ceres to spin itself apart".

This was also with the assumption that rockets were being added to the surface, with enough mass to spin Ceres. That mass would cause Ceres to slow down its spin, destabilizing its gravity well, and the rockets would attempt to spin Ceres in that destabilized condition.

[Suicide Junkie]

Reducing spin DOES NOT decrease the gravity by any measureable amount.
The rate at which you add mass means absolutely nothing.

The spin-rockets would have to be landed on the surface carefully, which means matching the spin rate of the surface so as not to plow into the asteroid and leave a crater full of expensive shrapnel. Installing the rockets would not change the spin rate of Ceres, and would increase its mass and angular momentum by an unnoticable fraction.

---

Unfortunately, searching for '"Ning Li" site:ua.edu' turns up zero items.
Ning and Li appear separately, though, as different people.

And if you open it to non - University of Alabama sites relating to spin and gravity, you get a buttload of crackpots, alien/UFO sites and such.

I'm sorry, but Newton's laws are the way it is, to great precision on the scales we are talking about.

Again, this is simple grade 9 physics.
Learn the basics, please.

[dogscoff]

inigma said:
That 0.03G is what is holding Ceres together. Force Ceres to spin at a 1G centripetal rate, and you can imagine the concequence.

I'm not sure that's the case. What's holding it together is not gravity. Gravity brings the material together in the first place and maybe later compresses it into denser material, but once it's all in place it's the atomic bonds between the atoms making up the rock that keep it from expanding to fill the vacuum around it. Otherwise, small rocks floating alone in space with negligible gravity would spontaneously vapourise the moment they got a sniff of a gravity well, and we know that don't happen.

Here's an experiment you can try at home: Take a big lump of rock. Drill a hole into it and then firmly screw or cement a hook into the hole. Use the hook to suspend the rock from above. Does the rock split in two, with the bottom half shearing away and crashing to the floor because of the 1G force pulling it downwards? Your hook might well pop out, depending on the weight of the rock and your metho of fastening, or the chain or crane might break, but the rock will be just fine.

Anyway, we established some time ago that we don't actually need to alter Ceres' spin at all, just its orbit. It's the coin surrounding Ceres that needs to spin.

And yes, you are right about the amazing properties required of the materials required to build this thing. I stated in an earlier post that such a material might be beyond credibility. However I have a feeling that it might work for smaller coins- perhaps only significantly smaller than Ceres, i don't know (Hell, you could you could use glass and steel up to a kilometre or ten, I reckon)- but I know for a fact that vast, Banksian structures would require bucketloads of unobtainium.

All that said, read up on carbon fullerenes. They have the potential to be insanely strong, and (I believe) transparent too.

[Suicide Junkie]

dogscoff said:
And yes, you are right about the amazing properties required of the materials required to build this thing. I stated in an earlier post that such a material might be beyond credibility. However I have a feeling that it might work for smaller coins- perhaps only significantly smaller than Ceres, i don't know (Hell, you could you could use glass and steel up to a kilometre or ten, I reckon)- but I know for a fact that vast, Banksian structures would require bucketloads of unobtainium.

All that said, read up on carbon fullerenes. They have the potential to be insanely strong, and (I believe) transparent too.

All you gotta do is make the coin spin slow enough for your materials . Depending on how much water you want to put in it, that might be significantly less than 1g.

However, even a small fraction of 1g should be sufficient to keep your environmental bits sorted properly. Fishing will certainly be interesting when the bass can jump out of the water and land halfway around the world