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Old September 14 2012, 04:38 PM   #13
Crazy Eddie
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Re: About planetary gravity

RB_Kandy wrote: View Post
Tiberius wrote: View Post
RB_Kandy wrote: View Post
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).
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