# Attractive equation

#### mitch bass

When determining the force of gravity that a planet exerts, is the speed of rotation a factor? If so is this because of centrifugal force? Yet is it not so that on a space station that creates centrifugal force, the attraction created is considered artificial gravity?

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#### russ_watters

Mentor
Yes, it has an effect, but the effect is very small (I can't remember offhand how small exacly). And yes, it works the same way as on a space station (except of course you are inside, not outside).

#### HallsofIvy

Homework Helper
No, the gravitational force does NOT depend in any way upon the rotation of a planet.

It is true that when you stand on a scale the reading is slightly LESS than the actual force of gravity: the "centrifugal force" directed away from the center of the planet offsets the gravitational force very slightly. However, it is still true that the actual gravitational force depends only on mass and distance from the center of the planet.

#### russ_watters

Mentor
Good clarification. I missed that one.

#### Janus

Staff Emeritus
Gold Member
Originally posted by mitch bass
Yet is it not so that on a space station that creates centrifugal force, the attraction created is considered artificial gravity?
I wouldn't call it artificial gravity but simulated[i/] gravity. Locally, it behaves like a lot like gravity, but has a different origin.

#### StephenPrivitera

Originally posted by HallsofIvy
the "centrifugal force"
Sounds vaguely familiar. What causes it?

#### wimms

Originally posted by HallsofIvy
No, the gravitational force does NOT depend in any way upon the rotation of a planet.

It is true that when you stand on a scale the reading is slightly LESS than the actual force of gravity: the "centrifugal force" directed away from the center of the planet offsets the gravitational force very slightly. However, it is still true that the actual gravitational force depends only on mass and distance from the center of the planet.
Although perception of gravity on surface is one thing, I got a question from this angle:
mass is a form of energy, thus density of energy causes gravity. But rotational momentum of planet also stores some kind of energy. Does that rotational momentum add to gravity or not?

#### HallsofIvy

Homework Helper
Well, yes, in that sense, increased energy add a little mass: have you calculated the mass added by the rotation of a planet?

It is, of course, m= E/c2. I think you find that negligible.

By the way, when you talk about a space station using rotation as an "artificial gravity" you are aware that people walk around on the INSIDE of the station with their heads toward the center? You make it sound as if you thought they would be walking around on the outside as on the earth.

Also, you cannot use centriugal force to create a uniform force on a sphere- you have to have a cylinder so every one is the same distance from the AXIS of rotation.

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