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Old September 14 2012, 12:42 PM   #10
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.
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.
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