If a planet would spin faster , would it have a lower gravity pull?

[SoldierHair]

So , as i was spinning in my chair and let the centrifugal (i might be wrong , i seriously don't know physics) force pull my head outside of the chair this idea came to me.

I don't know physics i decided to ask here.

[drakesdoom]

Yes and no. All else being equal it would have the same gravitational force, however since you would be moving faster the greater centrifugal force would counter some of it. For example a spacecraft with no orbital velocity would still fall at the same rate and the same orbital velocity would be needed for a stable orbit. Geo Stationary orbit would change, closer I think.

[karolk]

As Drake already explained, the gravitational pull wouldn't change as the mass of the planet remains the same. The rotational speed of a planet would only affect things moving with the ground and (Slightly less) in it's atmosphere.

Edited May 3, 2013 by karolk

[3_bit]

But what about spinning at orbital velocity speeds?

[karolk]

But what about spinning at orbital velocity speeds?

Nothing would change as long as you don't enter the atmosphere as you are only dependent from the gravity pull of the planet while in orbit and spin doesn't directly affect it.

[DuckZero]

Having only done a masters-level module in General Relativity (and not particularly well) I'd say that the extra rotational energy would increase the strength of gravity, but I'm not that good at manipulating the stress-energy tensor to say exactly how much.

If I remember correctly, some binary stars have enough rotational energy do exhibit a measurable increase in gravitational strength.

[RatBeer]

i was wondering this just the other day

[magnemoe]

Yes and no. All else being equal it would have the same gravitational force, however since you would be moving faster the greater centrifugal force would counter some of it. For example a spacecraft with no orbital velocity would still fall at the same rate and the same orbital velocity would be needed for a stable orbit. Geo Stationary orbit would change, closer I think.

Yes outside the atmosphere it would be no difference, however the atmosphere would roughly follow the rotation speed and you will reach it while landing.

On ground you will weight less at equator than at the poles, if you want to orbit again it would take less energy going with rotation than against. All this effects exists on earth but would be far more noticeable on an planet who rotated faster.

[Person012345]

On the ground you would experience lower gravity, although the effect would be less the further from the equator you go. See Mesklin, a fictional supergiant planet with rapid rotation.

[nyrath]

Came to this thread hoping for a Mission of Gravity reference. Left satisfied. Thanks Person012345!

[Stochasty]

Having only done a masters-level module in General Relativity (and not particularly well) I'd say that the extra rotational energy would increase the strength of gravity, but I'm not that good at manipulating the stress-energy tensor to say exactly how much.

Other way around. Consider the surface gravity of a Kerr black hole. As you approach maximal spin, the surface gravity goes to zero. That said, I haven't actually looked at what happens for a rotating extended object, but I doubt it would be much different.

[NASAFanboy]

I really do not know. All I know is that you will get to orbit easier.

[K^2]

But what about spinning at orbital velocity speeds?

For starters, the planet would fall apart.

[rryy]

So what if a planet was spinning at escape velocity? What would happen?

[Pyotor Gagarin]

But what about spinning at orbital velocity speeds?

It's worth noting, that any planet whose equator exceeded it's orbital speed wouldn't retain planet status very long. Mesklin still has a downward acceleration of three gees at the equator. (Much more everywhere else).

[Dunbaratu]

Then it wouldn't *be* a planet. Especially not one with a molten mantle. Magma doesn't have a lot of tensile strength.

(Also, it wouldn't form a planet in the first place if the matter was spinning rapidly enough to null out the effect of gravity. It's the matter pulling toward other matter that causes the planet to form in the first place.)

[Fractal_UK]

It's worth noting, that any planet whose equator exceeded it's orbital speed wouldn't retain planet status very long. Mesklin still has a downward acceleration of three gees at the equator. (Much more everywhere else).

Indeed, how would such a planet form in the first place? Planet rotation is initially dictated by the angular momentum of a cloud of coalescing material from a protoplanetary disc. If the rotation speed is higher than the escape velocity, then I guess you'd temporarily have some kind of vortex within the disc but it would certainly be a transient phenomenon. Planet rotation could be influenced later by impact events but it would take an absolutely ludicrous number of these, all of which would have to be aligned in the same direction or some impact events would cancel out the effects of others.

[Pyotor Gagarin]

any planet whose equator exceeded it's orbital speed wouldn't retain planet status very long

I don't honestly think a planet could form in those circumstances. Just an attempt at humor.

Since we're on the subject though,

As I understand it, if a planet has a moon which orbits faster than the planet rotates, angular momentum will be dumped inward. The moon will move closer and the planet will rotate faster and faster over time. The moon eventually breaks up.

In my layman's understanding, it seems possible that the debris field of a comparatively large moon could cause a planet to rotate itself apart temporarily as they combine to form a single planet. (Possibly a special case for the "orbiting equator" sci-fi story?)

Edited May 4, 2013 by Pyotor Gagarin

[metaphor]

You would be able to land and take of from it with much less delta-v as long as you were near the equator.

And the force pushing you down would be less on the equator than on the poles.

[GoldenShadowGS]

What effects would a high planetary revolution have on the terrain and atmosphere? Assume the planet is spinning very fast, but not fast enough to fling rocks into the sky. A space elevator would be much easier to build as the height to geostationary orbit would be much lower. The atmosphere would likely be less dense or bleed off into space. Mountains may be taller than if the planet was spinning slower.

[Nibb31]

You would have some more turbulence in any fluids due to the Coriolis effect, which would translate into stronger winds and ocean currents. It wouldn't affect gravity, because the mass of the planet would remain the same. The centrifugal force might cause a slight bulging around the equator.

The day/night cycle would be much shorter, which might translate into less temperature variations. Not sure what effect that would have on the climate.

[Kerbface]

I could be wrong, but I think what the OP meant to ask, though perhaps not in the correct terminology, is that would the forces of acceleration towards the Earth be diminished by the rotation. Not that the gravitational pull would physically be altered, but would the net force you feel change? I would imagine that under any plausible planet the effect would be minimal, but in a hypothetical indestructible planet put into a very fast rotation by magic, I would think the centripetal force could make the gravity seem to be significantly weaker. But also you might experience some noticeable Coriolis effects.

[NovaSilisko]

If I'm not mistaken, I think KSP actually emulates the centrifugal force on the surface of a planet. Experimentation will certainly be done when we make the rest of the solar system in <INSERT TIME>

[dustinsonger]

If I remember correctly, some binary stars have enough rotational energy do exhibit a measurable increase in gravitational strength.

I believe this is due to the velocity they have around a central gravitational point, not the rotation of the stars themselves. We know from special relativity that as velocity increases mass does as well.

[metaphor]

As an example, the Earth rotates at 465 m/s at the equator. This translates into a centrifugal force of 0.03 m/s^2, or 0.003 g's. So if the Earth were not rotating, you would feel 0.3% heavier at the equator.

Jupiter rotates at 12600 m/s at the equator, which is a force of 2.3 m/s^2, or 0.23 g's. Considering the gravity on Jupiter is 2.4 g's, if Jupiter were not rotating, you would feel 10% heavier at its equator.

Saturn is an even better case, the centrifugal force at the equator is 20% of the planet's gravity. This is why Saturn appears noticeably squashed in photos.

On the dwarf planet Haumea you would feel 14 times heavier on the poles than on the equator.

Edited May 4, 2013 by metaphor