Conceptual Questions About Space

Originally posted by CLPrime

Originally posted by Frira
If the Earth could remain the same distance from the sun and of the same mass, but be twice as large (and so appear larger from the sun), the sun's gravity has more surface area to attract, and so the force of the attraction would increase.

Gravity acts on mass, it doesn't act on surface area. If Earth were twice the size but the same mass, the gravitational attraction between Earth and the sun would be the same as it is now.
For gravity to increase, mass has to increase.

Hey Prime! Always nice running into you!

What you wrote is exactly what I had thought, but I adjusted that based on something I read recently (but may have misunderstood).

Before I began looking for a reconciling answer, my thoughts went like this:
.
* Since the gravitational force of an object is a function of the inverse square concerning distance;
* Since the angular size of an object is a function of the inverse square concerning distance;
* Since the sun and the moon are approximately of equal angular size from the earth; and
* Assuming the sun is at least as dense as the moon,
* Then, the sun should have at least equal gravitational effect on the earth's tides.

But, I read that the sun has not even half the tidal effect. So, I started digging for why.

I found that the surface area presented to the gravitation force is also a consideration-- this may be conceptualized as what percentage of the gravity force of a mass is at work within the angle the object upon which it acts encompasses.

From the surface of the sun, the Earth represents a small disk (nearly a point) of reflected light and the sun's gravity has corresponding slight effect across that surface; whereas from the surface of the moon, the Earth represents a substantial disk and so substantial effect is noticed on the surface of the Earth.

And I liked that answer-- because it allows a better understanding of gravity having effect on all parts, not just the gravitational center-- thus the oceans have tides.

Now you are going to complicate my understanding, aren't you?

reply to post by Frira

Now you are going to complicate my understanding, aren't you?

Would I do that?

What you wrote concerning the tides is exactly right, but it's a bit different than the case of a planet. The oceans are not a single unit with a specific center of gravity, and, so, their surface area is a factor... but this is only insofar as each unit of surface area is approximately proportional to a corresponding center of gravity (since, as I said, the oceans are not a single unit - they effectively have many "centers" of gravity).
The earth, on the other hand, is a single unit... no part of the earth can move independently of the rest. Thus, the earth has only one center of gravity.

So, while surface area does come into play with the tides, it's still an effect of gravity acting on mass. It's just that the oceans are not a single unit of mass.

I should also add that the effect is even more pronounced than your estimation suggests. As ErtaiNaGia, below, calculated, the gravitational force between the earth and the moon is about 2*10^20 N. The gravitational attraction between the earth and the sun is about 3.5*10^22 N. The attraction between the earth and the sun is actually over 100 times that between the earth and the moon. Yet the sun affects the tides, as you say, less than half as much as the moon does. This is a result of the relative surface area presented to each, as stated.

reply to post by TupacShakur

Gravity- According to Einstein's Theory of Relativity, massive objects bend the fabric of space time, and that is how planets and other stars orbit them.

Actually, we don't really need to go into relativity to explain orbital mechanics... and it's easier to visualize as a gravitational gradient.

Gravity, as a force (in terms of acceleration in meters per second, or newtons) is determined by the mass of the two objects, the separation distance between them, and the gravitational constant of the universe.

The equation is as follows:

Force of attraction = ((M1 * M2) / Distance squared) * gravitational constant

The Gravitational constant is 6.67 * 10^-11 Or: 0.0000000000667

The Mass of both objects is in kilograms

The distance is in meters


So, the attraction between the earth and the moon is as follows:

Earth's Mass: 5.9742 × 10^24 kilograms (5,974,200,000,000,000,000,000,000 kg)
Moon's Mass: 7.36 × 10^22 kilograms (73,600,000,000,000,000,000,000 kg)
Gravitational Constant: 6.67 x 10^-11 (0.0000000000667)
Separation distance: 384,403,000 meters

So, the attraction force between the earth and the moon is:

0.0000000000667 * ((5,974,200,000,000,000,000,000,000 * 73,600,000,000,000,000,000,000) / (384,403,000^2)) =

1.98476855 × 10^20 newtons (198,476,855,000,000,000,000 newtons)

a Newton is defined as the amount of energy it takes to accelerate one kilogram of mass by one meter per second in one second.

So, 2 newtons would accelerate 2 kilograms by one meter per second in one second, or one kilogram by 2 meters per second in one second.

Now, the thing that you have to realize, is that the gravitational attraction is acting upon both the moon, and the earth equally.

Meaning that the 198,476,855,000,000,000,000 newtons is acting upon the earth and the moon in equal portion.

The moon would naturally accelerate faster, because there is less mass to move.

[atsimg]http://files.abovetopsecret.com/images/member/59607b11fa02.gif[/atsimg]

The thing about the Earth moon system, is that the Earth and Moon are both actually orbiting around the Gravitational Barycenter, which is located at the "Gravitational Halfway point" between the earth and moon (which is probably still somewhere within the earth, but not at the center)

Since the moon has a velocity that is adjacent to the force of gravity, it is essentially FALLING towards the earth, but constantly missing it.

[atsimg]http://files.abovetopsecret.com/images/member/c20ab83aa1b3.gif[/atsimg]

This is Newtons theory of universal gravitation, and kepler's laws of planetary motion.

en.wikipedia.org...

but since there are three dimensions and in space there is no up, down, left, or right, what determines the plane that the planets orbit around?

Their initial velocity.

Picture a giant cloud of space dust...

Okay, now the dust starts clumping together because of gravity, and when it finally sticks, it actually still has kenetic energy, and thus contributes to the total angular velocity of the center mass.

Like when you are spinning in a chair with your arms outstretched, and then you bring your arms in close to your body, you actually spin faster, because the angular momentum is conserved.

IT's the same thing, actually....

Anyways, the accretion disk (the dust spinning around the sun that eventually formed our planets) finally accreting into mass bodies that were significant enough to have appreciable gravitational force, eventually cased dust to start orbiting around them, and when those orbits finally intersected the mass, they added their angular velocity TO that mass.

Wouldn't the bending of space time be uniform in all directions as opposed to straight below the object as seen in the above image?

IT is, but during Star formation, you would get a single uniform direction of motion because of the above explanation.

And judging by the bend in space time caused by the mass, how do the planets all stay on the same plane despite the fact that the warping of space time is more severe the closer the objects are towards the sun?

Actually, this is not nessecarily the case, as most planets are in ABOUT the same plane, they are not all perfectly lined up.

This difference between a planets orbital plane, and the plane of the suns rotation is known as Inclination, of which the Earth is about 7 degrees off.

en.wikipedia.org...

All planets have SOME inclination.... but it is generally rather smallish.

Also since an object with more mass has more gravity, do very small objects still warp the fabric of space-time, but just very slightly?

All mass has gravity.

But gravitational attraction increases with an increase in mass, and/or a decrease in distance.

This is where my mind begins to melt: If the space in between them is expanding, then why are some galaxies moving closer together?

Oh, this one is easy.....

When you have an explosion, like a grenade or something.... picture every particle in that explosion...

Since the explosion is expanding, MOST of the particles will be moving away from other particles...

However, a small number of these particles will be moving closer to other ones.

The observation of "The Expansion of Space" is merely the observation that most things in the universe are moving away from US.

Hubble isn't looking at "Space" itself, but objects IN space.

There's another part of the expansion of space that I can't understand: if the farther out you look, the faster things are expanding, then how does that not mean we are at the center of the universe?

Because according to the Big Bang Theory, EVERYTHING was once at the "Center" of the universe, and therefore, Everything is expanding away from everything else.

The expansion would pretty much appear uniform from all places in the universe.

I know we're not at the center of the universe obviously, but if the expansion speed is greater the farther out we look regardless of the direction....how does that work?

[atsimg]http://files.abovetopsecret.com/images/member/30955eab4e4b.jpg[/atsimg]

Whenever animated representations of black holes are made, they're almost always two dimensional holes in space that suck matter in.

Black holes are 3 dimensional objects in space.

How would matter be sucked into a three-dimensional black hole?

Well, Matter is actually mostly empty space.

The atoms that compose your body are electrons orbiting around a nucleus...

The scale of comparison is something like.... a grain of rice in the middle of a football stadium, with the stadium walls being the electron shell, and the grain of rice being the nucleus.

Anyways, the reason that YOU are the size you are, is because these electron shells actually repel each other.

and chemical bonds are basically just the electron shells butting up against eachother.

In a black hole, the force of gravity is FAR greater than the electrostatic repulsion of the electron shells, and it compresses all the matter together.

So, in a black hole, you have the same amount of mass that you had in the beginning, but you have gotten rid of almost all of that empty space.

A "Gravitational Singularity" (which is another term for a black hole) is basically a mathematical model where all of the Protons, Neutrons, and Electrons have all merged into a single.... "Particle"

or something like that....

And if black holes suck matter in because the gravity is so strong that not even light can escape, then what would the fabric of space-time look like underneath a black hole?

Meh... all we got at this point is mathematical theories.

But that is why they call it a "Singularity" because mathematics kind of breaks down at those levels.

hope this helped!

reply to post by CLPrime

Very good-- knew I could count on you!

While I wanted to bring in the OP's question concerning gravity in three dimensions; I neglected to indicate that the example I gave will have no effect on the planer trajectory of orbiting bodies.

To that end, I'll suggest that the models provided in the OP of the warped space of one plane, need to be understood to include that the surface is assumed to be frictionless, so it is that the planet stays on a plane within the warp, unlike the behavior of a marble rolling inside the hyperbole. That is to say, that the marble model would include a constant speed, keeping it on a plane.

In the gravity well demonstration using a marble (as is often seen in science museums), there is another neat parallel model which was used to teach astronauts the general concepts of "slow down = lower orbit, higher rate around the center of gravity; speed up = higher orbit and slower rate around the center of gravity." The test used the inverse of the concept for the astronauts: For a constant velocity... Lower = faster; higher =slower.

Probably in Tom Wolfe's The Right Stuff (or some such book-- I read so many like that) is the following concept demonstration actually used as I remember it:

Two Jeeps on a salt-flat with circles inscribed around the center point (probably a bomb target, but I don't recall the details). Each Jeep is maintaining the same constant speed. One Jeep drives along one circle's path. The other Jeep's job is to "rendezvous" by touching its front bumper to the rear bumper of the one holding a constant circular path. To catch-up with the Jeep on the outer circle, the maneuvering Jeep must circle inside, since it cannot change velocity.

Soon after that demonstration, we had two Gemini craft orbiting the Earth, station-keeping inches from each other for some time. I just think that is so cool.

Since the orbits were not perfectly circular, that feat is all the more amazing to me. I always wished there was computer game to let you experiment with various tactics to solve the problem.