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RB_Kandy September 14 2012 05:45 AM

About planetary gravity
 
I recently found out that gravity on any given planet is caused by the mass of the planet.

I had always assumed that gravity on a planet was caused by the speed of rotation, orbit, proximity to the sun, and size of the sun.

I began exploring why gravity didn't get stronger during its ellipse when it was closest to the sun, and less when it was far. It was googling that which lead me to realize the size of the planet was the cause of gravity.

But I am wondering, does speed of orbit and rotation play any part of planetary gravity?

RobertVA September 14 2012 06:04 AM

Re: About planetary gravity
 
There would be a minor weight decrease and change of net angle at low latitudes due to a planet's rotation, which would be in inverse proportion to the rotational period. Those forces would also tend to make the planet bulge a little around its equator.

There's also a daily tidal fluctuation, although for Earth the moon's proximity produces much more noticeable tidal effects. Tides produced by the system's star would be more powerful with a more massive star or a smaller distance between the star and the planet (as would be the case with Mercury)

Tiberius September 14 2012 06:57 AM

Re: About planetary gravity
 
Quote:

RB_Kandy wrote: (Post 6958870)
I recently found out that gravity on any given planet is caused by the mass of the planet.

I had always assumed that gravity on a planet was caused by the speed of rotation, orbit, proximity to the sun, and size of the sun.

I began exploring why gravity didn't get stronger during its ellipse when it was closest to the sun, and less when it was far. It was googling that which lead me to realize the size of the planet was the cause of gravity.

But I am wondering, does speed of orbit and rotation play any part of planetary gravity?

No. Technically speaking, gravity is caused by the density of a planet. If you had a planet with the same mass as Earth (the same amount of material), but spread out to the size of Jupiter, the gravity would be a great deal less, because it is less dense. However, if the planet had the same mass as Earth but was only the size of the moon, it would be denser, and therefore have a higher gravity.

RB_Kandy September 14 2012 07:16 AM

Re: About planetary gravity
 
Quote:

Tiberius wrote: (Post 6959008)
Quote:

RB_Kandy wrote: (Post 6958870)
I recently found out that gravity on any given planet is caused by the mass of the planet.

I had always assumed that gravity on a planet was caused by the speed of rotation, orbit, proximity to the sun, and size of the sun.

I began exploring why gravity didn't get stronger during its ellipse when it was closest to the sun, and less when it was far. It was googling that which lead me to realize the size of the planet was the cause of gravity.

But I am wondering, does speed of orbit and rotation play any part of planetary gravity?

No. Technically speaking, gravity is caused by the density of a planet. If you had a planet with the same mass as Earth (the same amount of material), but spread out to the size of Jupiter, the gravity would be a great deal less, because it is less dense. However, if the planet had the same mass as Earth but was only the size of the moon, it would be denser, and therefore have a higher gravity.

I can't say that you're wrong, but to me, it just sounds wrong, it goes against my understanding of physics in general (though it is easily argued my understanding of physics is lacking).
Here on earth, an object's weight is the total mass being pulled by earth's gravity. But you're telling me earth's gravity is not established by mass; but by density? It just doesn't make sense.

Alidar Jarok September 14 2012 11:26 AM

Re: About planetary gravity
 
Gravity is a strange force. It's powerful enough to keep a giant chunk of rock rotating around the earth, but it's not powerful enough to prevent me from jumping into the air.

It's clear gravity isn't caused by the Sun. Otherwise, we'd get sucked to the sun instead of staying on Earth. Likewise, Jupiter wouldn't have more gravity than us because their farther away from the sun and travel slower around it. However, their gravity is strong enough to attract dozens of moons and would be crushing if we were to enter Jupiter's atmosphere.

Maurice September 14 2012 11:36 AM

Re: About planetary gravity
 
Think of it this way: an Earth-sized body made of the stuff the Earth is has 1g of gravity. An identically sized body made of collapsed star material is a white dwarf and about 200,000 as dense. So two objects of the same size only have the same mass if they are are made up of material of the same density.

Or try this...
The Sun has ~333,000 times the Earth's mass
  • On the surface of an object with the Sun's mass that is the size of the Sun, you'd weigh ~28 times what you do on Earth
  • But on the surface of an object with the Sun's mass that is the size of the Earth, you'd weigh 333,000 times what you do on Earth.

Why?

Density.

Explained here in pretty simple terms (link).


Quote:

Alidar Jarok wrote: (Post 6959536)
Gravity is a strange force. It's powerful enough to keep a giant chunk of rock rotating around the earth, but it's not powerful enough to prevent me from jumping into the air.

That's because you're forgetting to factor a burst of acceleration, which gravity quickly overcomes and pulls you back down.

Quote:

Alidar Jarok wrote: (Post 6959536)
[Jupiter's] gravity is strong enough to attract dozens of moons and would be crushing if we were to enter Jupiter's atmosphere.

At Jupiter's "surface" (where gas becomes liquid) gravity is only 2.54 time Earth's. Ergo, we wouldn't be crushed by Jupiter's gravity. We'd be crushed by the insane atmospheric pressures there.

WalkerBait September 14 2012 11:38 AM

Re: About planetary gravity
 
Think of it this way. Imagine space as a semi-stretchy fabric spread across an open area until taut.

Imagine planets, stars, moons, etc. as different sized and weights of balls and such being placed on this fabric.

So, say, you had two balls that weighed the same but where different sizes. One the size of a baseball, one the size of a basketball and placed them on this stretchy fabric.

The baseball-sized object is going to cause a deeper "hole" in the fabric because it's more dense (takes up less space) than it's same-weighted counterpart, the basketball-sized object.

That "hole" or impression in the fabric is, essentially, what gravity is, and the fabric is the "material" of space. You just have to extrapolate our demonstration into all three dimensions at once.

Take a look at our sun, obviously it creates a lot of gravity. (28g's on the surface IIRC). But if we compressed the sun's mass into the size of Earth it'd be a miniature black hole. Because it's be a lot of mass in a relatively small space to the point it crated a very deep depression in the fabric of space.

(And counter to what Sci-Fi tells us, black holes do not "suck" things into them. They just have strong gravity. If the sun became a black hole the Earth (gravitationally speaking, at least) would remain un impacted. Though it might move closer, but not be "pulled in."

http://i1188.photobucket.com/albums/...47/Gravity.jpg

Deckerd September 14 2012 11:53 AM

Re: About planetary gravity
 
Quote:

RB_Kandy wrote: (Post 6959038)
But you're telling me earth's gravity is not established by mass; but by density? It just doesn't make sense.

Here you go: this cork and penny have the same mass (they weigh the same)

http://i1112.photobucket.com/albums/...d/density1.jpg

which has the higher density?

Maurice September 14 2012 11:56 AM

Re: About planetary gravity
 
Actually, the Sun's mass the size of the Earth would be a white dwarf, and crunched down to the diameter of Brooklyn would be a black hole.

Tiberius September 14 2012 12:42 PM

Re: About planetary gravity
 
Quote:

RB_Kandy wrote: (Post 6959038)
Quote:

Tiberius wrote: (Post 6959008)
Quote:

RB_Kandy wrote: (Post 6958870)
I recently found out that gravity on any given planet is caused by the mass of the planet.

I had always assumed that gravity on a planet was caused by the speed of rotation, orbit, proximity to the sun, and size of the sun.

I began exploring why gravity didn't get stronger during its ellipse when it was closest to the sun, and less when it was far. It was googling that which lead me to realize the size of the planet was the cause of gravity.

But I am wondering, does speed of orbit and rotation play any part of planetary gravity?

No. Technically speaking, gravity is caused by the density of a planet. If you had a planet with the same mass as Earth (the same amount of material), but spread out to the size of Jupiter, the gravity would be a great deal less, because it is less dense. However, if the planet had the same mass as Earth but was only the size of the moon, it would be denser, and therefore have a higher gravity.

I can't say that you're wrong, but to me, it just sounds wrong, it goes against my understanding of physics in general (though it is easily argued my understanding of physics is lacking).
Here on earth, an object's weight is the total mass being pulled by earth's gravity. But you're telling me earth's gravity is not established by mass; but by density? It just doesn't make sense.

Think of it like this. If you make the Earth less dense, you have to give it a larger volume - it will get bigger. But gravity pulls everything towards the center of the mass, which is why you don't get mountains pulling you sideways. This less-dense Earth is still pulling you towards its center, but because it is larger (in order to be less dense), you are now further from the center of gravity's pull, and gravity is pulling on you less.

Methos September 14 2012 03:22 PM

Re: About planetary gravity
 
Very simple way of explaining density...

Cannonball... very heavy chunk of metal right? So what happens when you drop it in a pool of mercury... mercury is a liquid, and cannonballs are pretty damn heavy...



easiest practical explanation of density i've ever seen :)

M

Chemahkuu September 14 2012 03:53 PM

Re: About planetary gravity
 
Quote:

RB_Kandy wrote: (Post 6958870)
I recently found out that gravity on any given planet is caused by the mass of the planet.

:wtf:

I don't know what they teach you over there but back here we knew that as soon as we were old enough to spell gravity.

Really?

Crazy Eddie September 14 2012 04:38 PM

Re: About planetary gravity
 
Quote:

RB_Kandy wrote: (Post 6959038)
Quote:

Tiberius wrote: (Post 6959008)
Quote:

RB_Kandy wrote: (Post 6958870)
I recently found out that gravity on any given planet is caused by the mass of the planet.

I had always assumed that gravity on a planet was caused by the speed of rotation, orbit, proximity to the sun, and size of the sun.

I began exploring why gravity didn't get stronger during its ellipse when it was closest to the sun, and less when it was far. It was googling that which lead me to realize the size of the planet was the cause of gravity.

But I am wondering, does speed of orbit and rotation play any part of planetary gravity?

No. Technically speaking, gravity is caused by the density of a planet. If you had a planet with the same mass as Earth (the same amount of material), but spread out to the size of Jupiter, the gravity would be a great deal less, because it is less dense. However, if the planet had the same mass as Earth but was only the size of the moon, it would be denser, and therefore have a higher gravity.

I can't say that you're wrong, but to me, it just sounds wrong, it goes against my understanding of physics in general (though it is easily argued my understanding of physics is lacking).
Here on earth, an object's weight is the total mass being pulled by earth's gravity. But you're telling me earth's gravity is not established by mass; but by density? It just doesn't make sense.

It's both, actually. Two things to understand here:
1) Gravity always pulls towards the exact center of a mass. So if you are in orbit around, say, an enormous gas cloud half a light year in diameter, you will be pulled towards the exact center of that cloud. If you then collapse that cloud into a point the size of a baseball, you are still being pulled towards the exact center of the ball.

2) Over short ranges at least, gravity is subject to the inverse square law. That means the farther you get from the center, the less its gravity affects you. From the nebula/ball example above, this means that the force of gravity half a light year away from the center is considerably less than it would be half a mile from the center. Just as important to understand, however, is that you can only be pulled in by the gravity of something BELOW you, so if you are INSIDE the gas cloud, you will not be affected by any mass that is farther away from the center than you are.

The second point is why density matters. If Earth 1 and Earth 2 have the same mass, but Earth 2 is half as wide, then Earth 2's surface gravity is greater than Earth 1. On the other hand, if you're 7000 miles from the center of Earth one (low orbit) you're subject to the same gravity force as you would be 7000 miles from the center of Earth 2 (slightly higher orbit).

Christopher September 14 2012 05:04 PM

Re: About planetary gravity
 
Density doesn't "cause" gravitation, it's simply a factor in determining the intensity of a planet's surface gravity.

Gravitation is a fundamental force felt between all masses. Any mass, even a single particle, exerts an attraction on every other mass around it, and vice-versa. But this is a very, very weak attraction, the weakest by far of the four fundamental interactions in nature, so only a really, really large concentration of mass, such as a star, planet, moon, or asteroid, exerts enough gravitational pull to be significant.

This gravitational force follows an inverse square law -- if you're twice as far away from something, you feel 1/4 the pull, and if you're 10 times as close, you feel 100 times the pull. Also, the mass of a given object behaves as if it's all concentrated at the object's center of mass. The pulls of parts of it in opposite directions from the center of mass cancel out, so the vector of attraction you feel toward an object is straight toward its center of mass, and you feel an acceleration equivalent to what you'd feel if all its mass were concentrated in a single point at the center. The reason you feel weaker gravity on a planet with lower density, therefore, is because that planet has a larger volume relative to its mass, and thus a larger radius. In short, you feel less gravity because you're farther away from the center of mass.

So no, gravity isn't "caused by density." That's confusing the concepts. It's caused by the presence of mass, and the magnitude of attraction between two masses is determined by the distance between them. Density is just a relationship between the mass and size of an object.

A planet's rotation does not generate its gravity. This is a surprisingly common misconception among the general public, and I think it's based on confusion with what we've seen in science fiction movies like 2001, showing space stations and ships generating "gravity" with rotation or spinning centrifuges -- an effect we experience in everyday life watching clothes spin in the dryer or riding certain carnival rides. But that's not literally gravity. It's an acceleration that creates a sensation of weight, equivalently to how the acceleration caused by the force of gravitation creates weight, but not generated in the same way. And rotational "gravity" pulls outward from the center, not in toward it. The Earth's rotation actually creates a slight outward (centrifugal) acceleration that cancels a tiny bit of the inward gravitational pull of its mass.

The size and proximity of the Sun has no effect on the amount of gravity/weight you'd feel on the surface of the Earth or Mars or any other planet. This is because planets are in orbit of the Sun, and orbit is free fall. That means that the planet and everything on it is literally falling toward the Sun, being pulled inward by its gravity -- yet is falling sideways fast enough that it curves around the Sun and never gets closer. Effectively, when you're in orbit, you're spinning fast enough that the outward, centrifugal acceleration is equal to the inward, gravitational acceleration, and they cancel out to zero -- so you don't fall into the thing you're orbiting and you don't feel any weight while you're in orbit.

So it doesn't matter how close you are to the Sun or how big it is; you don't feel its gravity when you orbit it. The only mass whose pull noticeably affects you is the Earth itself.

gturner September 14 2012 06:13 PM

Re: About planetary gravity
 
Perhaps we should keep things simple. Gravity is the force that attracts liquids to your keyboard.


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