How does Gravity work? (Now I wish I had paid attention to physical science in 9th grade but I was more of a English student. /shrug)

OK first let me explain my question a bit better. If I had a ball and I put water on it, in the weightlessness of space. Now I start to spin it, the water would be flung off. Right? If that ball was super dense, with the center denser than the surface, the same ratio as our planet just smaller. I would assume it would do the same thing regardless of its density.

So at some point, I assume, mass of an object generates gravity, but how? And, does the centrifugal(sp?) force of the planet repel us away from the surface to an extent that we aren't squished like ketchup packets under a bike tire, or is the centrifugal force creating the gravity that holds us in place? If it is the centrifugal force creating the gravity why aren't we flung from the planet?

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How does Gravity work? (Now I wish I had paid attention to physical science in 9th grade but I was more of a English student. /shrug)

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What is gravity? Actually that is a tough question because, the two main physics theories that explain the universe - Quantum Theory and General Relativity - have different, and contradictory, explanations. But it is easier to explain at a simpler way at the Newtonian level. (Gobbledegook, I know, but I'm getting to it - that was for the pedants amoung us.)

Gravity is the attraction between two masses. Every piece of matter is attracted to every other piece of matter. On Earth, the Earth is the biggest piece of matter around, so that is so much stronger than everything else close by, so it tends to dominate the show. But, if I wanted to, I could calculate the attraction between the two of us, simply by knowing our weights and the distance between us.

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OK first let me explain my question a bit better. If I had a ball and I put water on it, in the weightlessness of space. Now I start to spin it, the water would be flung off. Right?

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Right. But before we go any further, there is a concept you will need to know, called inertia. Inertia is the tendency for a body to stay moving at the speed that it already is. And that is in a straight line - not spinning.

The water on a spinning ball issue works like this: The water is held to the ball by a different force to gravity - surface tension. Although there is some gravity, the surface tension is so much stronger than the gravity that we can almost ignore the gravity. When the ball spins, the water wants to keep going in the same direction (inertia), but the surface tension keeps the water to the ball by forcing it into the ball i.e. inertia vs surface tension - surface tension wins the battle. If it spins fast enough, then the intertia wins the battle, and the water flies off.

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If that ball was super dense, with the center denser than the surface, the same ratio as our planet just smaller. I would assume it would do the same thing regardless of its density.

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Density is how much mass (measured in kilograms) is found in an specified volume. Copper is denser than aluminum, and gold is denser than copper etc. But if you had a basketball made with a denser material than the Earth, (which is mainly iron) then the gravity would still be pretty small.

However, super dense materials can be found in neutron stars, white dwarves etc. These materials are so dense, that a teaspoon of them weighs many tonnes. That is very different to what is in the Earth. (You may be getting mixed up between density and pressure.) A basketball of this type of material would have significant gravity - enough for you to feel. The water would not likely fly off that unless it was spinning extremely fast.

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So at some point, I assume, mass of an object generates gravity, but how?

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Quantum theory suggests gravitons are pushing the particles towards each other. General Relativity suggests that space-time is warped by mass, and the "force" is just the mass moving where it would have gone. Effectively, this says that gravity is not a force, per se, but a warping of space-time.

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And, does the centrifugal(sp?) force of the planet repel us away from the surface to an extent that we aren't squished like ketchup packets under a bike tire, or is the centrifugal force creating the gravity that holds us in place? If it is the centrifugal force creating the gravity why aren't we flung from the planet?

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See the explanation above. Centrifugal force is not a real force, at least not in the same way that gravity is. Centrifugal force is the force of everything else trying to stop you moving in a straight line (inertia). If it really existed, then you would feel it when standing next to a spinning body - which you don't. In a gravity machine at an amusement park, you think you feel centrifugal force because your inertia is trying to push you through the seat.

F = G*M*m / r^2

The mutual attractive force between any two objects, due to gravity, depends on:
M,m - the mass of each object (more mass, more force)
r - the distance between the two objects (more distance, less force)
G - a constant of the universe

The thing to remember about gravity, is that if we work in units of kilograms and meters, G is ~10^-10, that's 0.0000000001, so unless you're talking about REALLY massive objects, gravity just isn't going to take much of an effect over the other forces of nature (electromagnetic forces, nuclear forces)

Thats why, most of the time, we mostly talk about gravity due to planets and stars and really massive astrophysical objects. Just to throw some numbers at you, even if you take something as dense as a neutron star (roughly the mass of the sun but ~ 50,000 times smaller) and create a ball of 1 cubic meter of neutron-star-stuff, the acceleration towards it that you feel from 1 meter away is still only going to be about the same as the gravity you feel on the surface of the earth, and we're talking about some of the densest stuff in the universe.

Anyway, that was probably too mathy for most, so I apologize. In short, gravity only really does anything with really massive objects.

In a purely empty universe, two atoms at opposite sides of the universe, will be attracted to each other and EVENTUALLY come together because of gravity. We are not in an empty universe, but if you bring things close enough, local gravity effects will overwealm distant ones.

I have used in many story ideas the concept that mass in our dimension, equals volumn in the next dimension. particles coming from outside the universe pass through in all directions applying pressure to the volumn. When two objects block the flow between them, they are not pushed away from each other, so they end up being pushed together. The effect looks like gravity.
A lot of questions are unanswered. I mainly use this as a way to get a form of faster than light travel using saling ships in this dimension.

For usefullness in a story, just go with what others have stated. Why is not usually needed, just see the effect.

"Well, as you already know, Bob, gravity works because invisible little green fairies are all around us, trying to push everything together into one big lump of mass so the great dragon can eat it all at once. So I invented this machine that scares the fairies away for a while...."

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Just curious, could a writer then make up a new theory for how gravity works in a science fiction setting, so long as it was logical? (I'd say run it past knowledgeable people to be sure.)

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best suggestion is to read about some of the past and modern theories so you have a foundation of how to present it.

In my stack of story ideas I need to post, I have a note that says that everything is made of sawdust, even atoms.
I have not posted it because I don't have a "handle" on how to use it, a story around the "discovery" and results.

If you can make the theory sound at least plausible, if not possible, then good writing will cover the rest of it.

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How does Gravity work?

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Why are opposites attracted toward each other in a correlating ratio as their mass? I don't know. Newton "dare(d) not feign a hypothesis," and modern science is simply bamboozled, without anything more than a dozen-dozen baseless theories.

For example, the presence of a massive star warps spacetime, causing the path of a "straight line" too bend towards the star, in essence producing gravity for other massive objects. Now because of this warped spacetime, we have actually measured deviations in LIGHT from distant objects also being warped along their path, even though light has no mass, because, at its root, gravity is caused by the warping of spacetime, which will affect everything passing by, not just other massive objects.

The theories of general and special relativity are fairly sound, and I don't think they are wrong, by any means, but that doesn't mean they answer the question. There is always a deeper why, for instance, if matter affects space-time, why? What is it about matter, physically, that causes that? You see we notice behavior, we setup models to test it, we even get good about predicting it, and eventually we'll be able to write textbooks that explain how it dos what it does. But that doesn't mean we know why. Because honestly, we don't know why so much as how.

I was wrong to say "baseless theories" it's not like I believe matter is pulled together by angels, or something, I just wanted to point out that ask "what is gravity" is a very-very hard question. ANd I wanted to feel smart by pointing that out.

A short answer from a writers perspective: no one knows with any certainty gravity's mechanism, only its effect measured empirically. As a writer, feel free to offer any malarkey you please for the "how" of it, so long as your malarkey is consistent with the "what" that we've measured.

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Which is all a wonderful example of why I am so delighted that I write fantasy!

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Interestingly enough, I heard Michael Stackpole discuss something related to this comment just today. He gets very frustrated by fantasy writers who think that they are excused from doing research by virtue of writing fantasy, pointing at that excuse as a major reason there is a lot of crappy fantasy out there.

I'm curious because, even though I don't read much fantasy--and--I don't write it much either, I've never read a particularly bad passage and thought to myself "This is ridiculous! The magic is totally unbelievable!" or "Oh please... a race like that would never evolve," I think, by the very nature of fantasy, I expect things to be somewhat... er... fantastic in nature.

However, I did read Paolini's Eldest and distinctly remember rolling my eyes at the very notion that a bunch of rag-tag villagers, armed with masonry tools, could easily dispatch a similarly-sized army of professional soldiers with swords. But I don't blame that on a lack of research so much as rampant silliness.