B Gravity of the Earth decreases if it stops spinning?

[aiop]

Would the gravity of Earth decrease if the Earth stop spinning? Would the gravity be affected at all? Does the gravity of other orbiting bodies cause the Earth to speed up its rotation and slow down its rotation during the year? thanks.

 

[phinds]

Would the gravity of Earth decrease if the Earth stop spinning? Would the gravity be affected at all?

Why would you expect it to? What are the factors that affect gravity?

 

[aiop]

Why would you expect it to? What are the factors that affect gravity?

The mass effects gravity but when you spin an object it creates a force going towards the center cause of centripetal force. So wouldn't that contribute to the force of attraction towards the center of the Earth the fact that its spinning ?

 

[phinds]

The mass effects gravity but when you spin an object it creates a force going towards the center cause of centripetal force. So wouldn't that contribute to the force of attraction towards the center of the Earth the fact that its spinning ?

Really? If you put a drop of water on a top and then spin the top, your theory says that rather than flying off off the top, the drop would flatten out against the surface due to attraction to the center.

Do you want to talk about gravity or do you want to talk about centripetal force? Do you think they are the same thing? I'm not asking if you think they both have an effect. They do. I'm asking which you want to talk about or if you think they are the same.

 

[aiop]

Really? If you put a drop of water on a top and then spin the top, your theory says that rather than flying off off the top, the drop would flatten out against the surface due to attraction to the center.

Do you want to talk about gravity or do you want to talk about centripetal force? Do you think they are the same thing? I'm not asking if you think they both have an effect. They do. I'm asking which you want to talk about or if you think they are the same.

I don't think there the same thing, yet isn't artificial gravity created by simply spinning a space station. And the Earth is spinning so would that create a force towards the center that would be like gravity or at least amplify its reach or effect. I don't think there the same but don't they both have the same effect spinning an object and having a object with mass cause both just have a force directed towards the center is that right ?

 

[phinds]

I don't think there the same thing, yet isn't artificial gravity created by simply spinning a space station. And the Earth is spinning so would that create a force towards the center that would be like gravity or at least amplify its reach or effect. I don't think there the same but don't they both have the same effect spinning an object and having a object with mass cause both just have a force directed towards the center is that right ?

You have not addressed my statement about the drop of water on a top. Do you really think a drop of water on a top would be flattened towards the surface due to a "gravity-like" attraction? How about a merry-go-round? If you stand on the edge of one and hang on are you going to be attracted towards the center?

 

[Drakkith]

I don't think there the same thing, yet isn't artificial gravity created by simply spinning a space station. And the Earth is spinning so would that create a force towards the center that would be like gravity or at least amplify its reach or effect. I don't think there the same but don't they both have the same effect spinning an object and having a object with mass cause both just have a force directed towards the center is that right ?

The force that is created by spinning a space station is directed away from the axis of rotation, not towards.

 

[aiop]

You have not addressed my statement about the drop of water on a top. Do you really think a drop of water on a top would be flattened towards the surface due to a "gravity-like" attraction? How about a merry-go-round? If you stand on the edge of one and hang on are you going to be attracted towards the center?

The water would fly off. So what your saying is the Earth spinning actually has the opposite effect of what i thought and it causes there to be a force that allows us to reach escape velocity easier or for there to be a force that pulls us off the earth.

 

[aiop]

The force that is created by spinning a space station is directed away from the axis of rotation, not towards.

Oh, Then how would artificial gravity be created ?

 

[Drakkith]

Oh, Then how would artificial gravity be created ?

It can't. Not in the way you are imagining. People would walk on the inside of a spinning space station, with their heads pointed towards the axis of rotation.

 

[aiop]

It can't. Not in the way you are imagining. People would walk on the inside of a spinning space station, with their heads pointed towards the axis of rotation.

So if the space station was a wheel spinning, you would walk on the inside of the rubber. You could wake around that well having the forces of artificial gravity apply to you ? Thanks.

 

[Drakkith]

So if the space station was a wheel spinning, you would walk on the inside of the rubber. You could wake around that well having the forces of artificial gravity apply to you ? Thanks.

Indeed.

 

[tony873004]

So wouldn't that contribute to the force of attraction towards the center of the Earth the fact that its spinning ?

Your weight is the centripetal force causing you to go in a circle. And it does contribute to the force of attraction towards the center of the Earth. In fact, it is the only force doing this. It is also doing this to the ground you're standing on. So in a rotating frame of reference, you perceive a loss of weight as the ground is being pulled from you.

 

[A.T.]

Would the gravity of Earth decrease if the Earth stop spinning?

At the poles.

 

[aiop]

At the poles.

Why at the poles but no where else? Wouldnt the effect of the Earth not spinning be the smallest at the poles cause there not really spinning very much to begin with.

 

[phinds]

Why at the poles but no where else? Wouldnt the effect of the Earth not spinning be the smallest at the poles cause there not really spinning very much to begin with.

You are missing the point. The spinning has NOTHING to do with gravity. What does affect the gravitational attraction felt by a body on the surface of the Earth is how far it is from the center. With the Earth spinning, the equator bulges out just slightly so the force of gravity there is slightly less that at the poles. If the spinning stopped, the Earth would, presumably, go back to a more spherical shape which means the poles would rise just a tiny amount and thus less gravitational attraction there.

 

[aiop]

Your weight is the centripetal force causing you to go in a circle. And it does contribute to the force of attraction towards the center of the Earth. In fact, it is the only force doing this. It is also doing this to the ground you're standing on. So in a rotating frame of reference, you perceive a loss of weight as the ground is being pulled from you.

You are missing the point. The spinning has NOTHING to do with gravity. What does affect the gravitational attraction felt by a body on the surface of the Earth is how far it is from the center. With the Earth spinning, the equator bulges out just slightly so the force of gravity there is slightly less that at the poles. If the spinning stopped, the Earth would, presumably, go back to a more spherical shape which means the poles would rise just a tiny amount and thus less gravitational attraction there.

Ok thanks for clearing it

You are missing the point. The spinning has NOTHING to do with gravity. What does affect the gravitational attraction felt by a body on the surface of the Earth is how far it is from the center. With the Earth spinning, the equator bulges out just slightly so the force of gravity there is slightly less that at the poles. If the spinning stopped, the Earth would, presumably, go back to a more spherical shape which means the poles would rise just a tiny amount and thus less gravitational attraction there.

Ok i understand that part now thank you! But does that mean your slightly lighter at the equator, and slightly heaver at the poles well the Earth is spinning because your father away from the center of mass at the equator because of the bulge in the Earth due to the centripetal force.

 

[phinds]

Ok i understand that part now thank you! But does that mean your slightly lighter at the equator, and slightly heaver at the poles well the Earth is spinning because your father away from the center of mass at the equator because of the bulge in the Earth due to the centripetal force.

Yes, that's exactly what I just said.

The big issue in this thread is your mistaken belief that centripetal force acts towards the center. Are you clear about that now?

 

[KarenRei]

Hi aiop. Sorry that phinds has been a bit short with you; I understood immediately what you were getting at. You simply were calling centripetal force "gravity" - simple mistake.

Earth's net "tug" at the equator is indeed lower than at the poles for two reasons. One is exactly, as you noted, centripetal force. But the other less obvious one is that things at the equator are further from the Earth's center of mass, as Earth is oblate (fattened) there due to its spin. It turns out that this latter factor has even more of an effect than the former.

If the Earth suddenly stopped spinning, well, obviously the surface would be in complete chaos due to the rapid deceleration ;) But even if you were to "damp" that slowdown so that every object (not to mention the oceans) didn't keep flying in the direction that they had been moving, you've still got a big problem: now there's no force propping up Earth's equatorial bulge. So the Earth will collapse into a new equilibrium stage, with mass steadily shifting from the mantle at the equator toward the poles. This process will involve a tremendous release of energy, and probably resurface the entire crust (aka, the worst series of volcanic events in Earth's history since the collision with the moon's progenitor!)

As for the other issue - "artificial gravity on space stations" - as was mentioned, that's not gravity either, that's just centripetal force. There's a really obvious difference with it, too - tidal forces. Imagine you're rotating in a circle twice your height. Your feet will be moving twice as fast as your head. Which means that they experience twice as much force, which means that tidal forces - that is, a difference in accelerations between different locations - will tend to pull the blood away from your head to your feet! Hence if you're using such an "artificial gravity" - again, stressing that it's not actually gravity - you need to have as wide of a radius as possible for crew comfort!

 

[A.T.]

Why at the poles but no where else?

I didn't say "nowhere else" but the reduction in gravity would be strongest at the poles. They would move away from the centre, when the stopped planet becomes spherical again.

 

[aiop]

Yes, that's exactly what I just said.

The big issue in this thread is your mistaken belief that centripetal force acts towards the center. Are you clear about that now?

Yes, thank you for the help ! :)

 

[aiop]

Hi aiop. Sorry that phinds has been a bit short with you; I understood immediately what you were getting at. You simply were calling centripetal force "gravity" - simple mistake.

Earth's net "tug" at the equator is indeed lower than at the poles for two reasons. One is exactly, as you noted, centripetal force. But the other less obvious one is that things at the equator are further from the Earth's center of mass, as Earth is oblate (fattened) there due to its spin. It turns out that this latter factor has even more of an effect than the former.

If the Earth suddenly stopped spinning, well, obviously the surface would be in complete chaos due to the rapid deceleration ;) But even if you were to "damp" that slowdown so that every object (not to mention the oceans) didn't keep flying in the direction that they had been moving, you've still got a big problem: now there's no force propping up Earth's equatorial bulge. So the Earth will collapse into a new equilibrium stage, with mass steadily shifting from the mantle at the equator toward the poles. This process will involve a tremendous release of energy, and probably resurface the entire crust (aka, the worst series of volcanic events in Earth's history since the collision with the moon's progenitor!)

As for the other issue - "artificial gravity on space stations" - as was mentioned, that's not gravity either, that's just centripetal force. There's a really obvious difference with it, too - tidal forces. Imagine you're rotating in a circle twice your height. Your feet will be moving twice as fast as your head. Which means that they experience twice as much force, which means that tidal forces - that is, a difference in accelerations between different locations - will tend to pull the blood away from your head to your feet! Hence if you're using such an "artificial gravity" - again, stressing that it's not actually gravity - you need to have as wide of a radius as possible for crew comfort!

Thanks for the answer :)!

 

[Buckleymanor]

You are missing the point. The spinning has NOTHING to do with gravity. What does affect the gravitational attraction felt by a body on the surface of the Earth is how far it is from the center. With the Earth spinning, the equator bulges out just slightly so the force of gravity there is slightly less that at the poles. If the spinning stopped, the Earth would, presumably, go back to a more spherical shape which means the poles would rise just a tiny amount and thus less gravitational attraction there.

With the Earth spinning, the equator bulges out so the force of gravity is slightly more than at the poles because there is more mass at the equator.
If the spinning stopped the Earth would eventually go back to a more spherical shape which would mean the poles would rise slightly and thus there would then be more gravitational attraction because there would then be more mass below them.
The reason why there appears to be less gravitational attraction at the equator if you weighed something there than at the poles is due to centripetal acceleration.
This also causes the bulge at the equator and the flattening at the poles which slightly counteracts the effect due to the re- distribution of mass.

 

[phinds]

If the spinning stopped the Earth would eventually go back to a more spherical shape which would mean the poles would rise slightly and thus there would then be more gravitational attraction because there would then be more mass below them.

Interesting. I hadn't thought about that aspect. I wonder if anyone has done calculations to compare the decrease in gravity due to rising surface with the increase in gravity due to your point.

 

[Buckleymanor]

Interesting. I hadn't thought about that aspect. I wonder if anyone has done calculations to compare the decrease in gravity due to rising surface with the increase in gravity due to your point.

I remember reading it somewhere possibly a slight lag " difference" between the two due to the elasticity of the Earth or the lack of it.
There are probably more accurate figures somewhere.

 

[Buckleymanor]

I didn't say "nowhere else" but the reduction in gravity would be strongest at the poles. They would move away from the centre, when the stopped planet becomes spherical again.

There would not be a reduction in gravity at the poles the gravity would increase as there would be now more mass between the poles and the centre of the Earth.
More mass equals more gravitational pull not less.
At the equator the reverse would be true once the planet stopped and became spherical again.
Over all the planet, gravity would be the same, due to it becoming spherical, the mass distribution should be the same in all directions, except for local anomalies.
For example areas where there is high density rock formations.

 

[A.T.]

There would not be a reduction in gravity at the poles the gravity would increase as there would be now more mass between the poles and the centre of the Earth. More mass equals more gravitational pull not less

But more distance means less gravitational pull, and that effect wins here. See:
http://physics.stackexchange.com/qu...ional-force-on-a-non-rotating-oblate-spheroid

 

[Aman Trivedi]

Would the gravity of Earth decrease if the Earth stop spinning? Would the gravity be affected at all? Does the gravity of other orbiting bodies cause the Earth to speed up its rotation and slow down its rotation during the year? thanks.

Nope. It wouldn't affect the Earth's gravity. Though the fact that the Earth is spinning at 1040 mph, therefore we are too. If the Earth did stop spinning everyone and (almost) everything around would fall, fly or crash till the point gravity doesn't begin to take over the catastrophic effects of inertia at that point. Hope this answers your questions :)

 

[Drakkith]

Nope. It wouldn't affect the Earth's gravity.

That is incorrect. Please read the replies in-thread before responding to a question.

 

[Buckleymanor]

But more distance means less gravitational pull, and that effect wins here. See:
http://physics.stackexchange.com/qu...ional-force-on-a-non-rotating-oblate-spheroid

There is more distance and therefore less gravitational pull but that effect does not win if the distance is filled with mass.
Which is what we are inquiring about.
Simply put more mass more gravitational pull. Which wins in this case so you are correct.
Tricky calculation but not totally convinced.

 

[stormrider919]

'More distance means less gravitational pull' can't imho be universally true though, but only within certain bounds. Because if you go to the extreme, then you can make an ellipsoid that thin that it approaches a thin disc. Now if you are standing on a pole of it, it means that you are really close to the center of mass, but have almost no mass below you. The way biggest part of the mass will be almost around you, so the gravity vectors going almost horizontal, cancelling each other mostly out at your position, only the small vertical component being left over and summing up. That way you can make the gravitational force arbitrarily small,,while still standing closer and closer to the center.

That's at least what I assume without doing the math. In the end you'd have to do the math and integrate and then maybe determine the ellipsoid shape with maximum gravity on a pole or something.

 

[A.T.]

there would be now more mass between the poles and the centre of the Earth.
More mass equals more gravitational pull not less.

Here is another counter example to the above:

http://pages.physics.cornell.edu/~aalemi/random/planet.pdf

In Figure 3, the flattened blue planet has stronger surface gravity at the top than the red sphere of equal mass and density, despite having less mass "between the poles and the center" than the sphere.

 

[Buckleymanor]

Here is another counter example to this:

http://pages.physics.cornell.edu/~aalemi/random/planet.pdf

In Figure 3, the flattened blue planet has stronger surface gravity at the top than the red sphere of equal mass and density, despite having less mass "between the poles and the center" than the sphere.

Very interesting though I find it hard to get my head around the fact that the flattened blue planet attracts matter with more force horizontally .
I was always taught that gravity works in the vertical plane more strongly and has little or no effect horizontally.

 

[A.T.]

I was always taught that gravity works in the vertical plane more strongly and has little or no effect horizontally.

You have to consider both: the direction, and the magnitude (which depends on distance).

 

[A.T.]

I was always taught that gravity works in the vertical plane more strongly and has little or no effect horizontally.

Here is Fig 3 from http://pages.physics.cornell.edu/~aalemi/random/planet.pdf with some force vectors added :

Point A is ~1.79 times further away from the top than point B, making B's direct attraction 3.2 times stronger. So despite B pulling mosty sidewards, it still creates ~1.43 times more downwards pull than A. Therefore by moving mass from A to B, you increase the surface gravity at the top.

 

[Buckleymanor]

Here is Fig 3 from http://pages.physics.cornell.edu/~aalemi/random/planet.pdf with some force vectors added :

Point A is ~1.79 times further way than point B, making B's direct attraction 3.2 times stronger. So despite B pulling mosty sidewards, it still creates ~1.43 times more downwards pull than A. Therefore by moving mass from A to B, you increase the surface gravity at the top.

So basically if the Earth stopped spinning the gravitational pull would be much the same over it's entire surface.
The gravity at the poles would decrease whilst at the equator it would increase as the planet became more spherical in shape.
Apart from it being much the same the rest is the opposite as to what I imagined thanks for the clarifications.

 

[BillyT]

I didn't say "nowhere else" but the reduction in gravity would be strongest at the poles. They would move away from the centre, when the stopped planet becomes spherical again.

We agree, I think, that the equatorial diameter would become less and surface gravity there slightly more, IF that were the only change (no relocation of ocean water) but why do you think the distance between the poles would increase, have reduced surface gravity at the poles? The Earth is not an incompressible solid, so the contraction at the equator, does not necessarily mean there must be expansion elsewhere (at the poles). Even if there were, what I discuss in next paragraph very likely lessens the distance between the poles, making surface gravity increase there.

As a great volume of water now near the equator would shift toward the poles; that increased polar weight might even reduce the separation between the poles, and make polar gravity slightly increase. Only thing for sure is the Earth would become on average slightly denser.* That lesser water mass at the equator would partially (or more than?) cancel out some of the "no-spin" contraction effect at the equator.

* Part of this density increase would take a few decades for the colder water, on average, to contract and fill a smaller volume.

 

[Joyousluo]

I think in contrast, gravity will be increased. Because the attraction force applied on us devides into 2 force, one is parallel to the surface and cause the friction between the ground and us. The other is the actual gravity, or weight we perceived. Hence when we launch a rocket, we often go to the equator to save fuel,because the linear velocity is highest and the gravity is smallest. Thus as u say when Earth stop rotating, the gravity will increase.

 

[Joyousluo]

I think in contrast, gravity will be increased. Because the attraction force applied on us devides into 2 force, one is parallel to the surface and cause the friction between the ground and us. The other is the actual gravity, or weight we perceived. Hence when we launch a rocket, we often go to the equator to save fuel,because the linear velocity is highest and the gravity is smallest. Thus as u say when Earth stop rotating, the gravity will increase.

 

[Clark Magnuson]

The rotational energy 1/2 I w^2 is equal to some additional Earth mass.
My seat of the pants guess is that is tiny when pushed through the E= mc^2 formula.

Whereas the centrifugal force closer to the equator is significant and is half of why we launch rockets from our Southern border.

 

[Buckleymanor]

I think in contrast, gravity will be increased. Because the attraction force applied on us devides into 2 force, one is parallel to the surface and cause the friction between the ground and us. The other is the actual gravity, or weight we perceived. Hence when we launch a rocket, we often go to the equator to save fuel,because the linear velocity is highest and the gravity is smallest. Thus as u say when Earth stop rotating, the gravity will increase.

If that were the case then a spinning top or gyro would weigh less when spinning and more when stopped, it does not though.

 

[Triple88a]

If the Earth stopped spinning you'd feel more gravity on the surface. Think about this for a second. Centripetal force negates some of the gravity we have from Earth's mass. If that centripetal force stops the only thing left will be Gravity because of mass.

 

[elenzil]

here's another angle - what about the gravitational effect of the energy of Earth's spin ?
ie, to change Earth from not spinning to spinning, one would have to put energy into the system. where does it go ? into the spin itself.
and e = mC², so from a gravitational point of view, a spinning Earth has more 'mass' than a non-spinning Earth by Δm = Δe/C².

but of course, as many people have mentioned, there are other effects at play which would possibly drown out this change in the force experienced by an observer on the surface of the planet,
but the change in gravity due to the energy of the spin might be measurable from a satellite in orbit.

 

[Triple88a]

The extra energy from the rotation is in direct relation to the centrifugal force.

 

[elenzil]

The extra energy from the rotation is in direct relation to the centripetal force.

right, and a satellite in orbit would not experience the centripetal force but would experience the gravitational force.

 

[looking4sophia]

The FORCE of Gravity we feel on the Surface is 'roughly' defined as 1 G.
It varies slightly from 1 G locally, especially around the Equator and Poles as discussed above by others.

Gravity is a 3D FIELD that varies
from ZERO G at the Center of the Earth
to One G at the Surface

1 G causes Mass to Accelerate at 32 Feet per Second per Second

This Diagram Measures only the GRAVITY in Meters per Second per Second
you would experience on a Journey to the Center of the Earth.
https://commons.wikimedia.org/wiki/File:EarthGravityPREM.jpg

Notice that Gravity Increases
as you dig down to almost half way to the center.

Then tapers off to ZERO G at the Earth's Center.

 

[ProfChuck]

Would the gravity of Earth decrease if the Earth stop spinning? Would the gravity be affected at all? Does the gravity of other orbiting bodies cause the Earth to speed up its rotation and slow down its rotation during the year? thanks.

Actually this is a more interesting question than it first appears. A rotating massive body possesses its intrinsic mass plus the mass of the energy-mass equivalent as derived from e=mc^2. Considering this the gravitational curvature would be less for a non rotating object than for a rotating one. Now this is a general relativity consideration and goes beyond special relativity. Anyway, there is also the Lens-Thirring effect which alters things like the precession rate of a gyroscope in the vicinity of rotating masses among other phenomena. This question deserves some careful consideration.

 

[John Green]

A number of respondents here seem to have centripetal and centrifugal forces confused. First of all centrifugal and centripetal are words that have fundamental meanings outside of physics. Centripetal (Moving or directed toward a center or axis.) e.g. Physiology Transmitting nerve impulses toward the central nervous system ;afferent.
Centrifugal (Moving or directed away from a center or axis.) e.g. Physiology Transmitting nerve impulses away from the central nervous system ;efferent.

Thus when one understands that these words are descriptive one is ready to apply them to physics. So a centripetal force is one that acts toward a center, thus gravity can in this sense be described as a centripetal force while the (apparent) force produced by rotation can be considered a centrifugal force.
So the straightforward answer to the original question (disregarding relatively minor effects like the accommodation of the Earth's shape which would probably take thousands of years(Canada is still rebounding from the weight of the ice sheet 10,000 years ago) ) is that if the planet stopped spinning the force of gravity would not change. The measured weight of any mass however would increase because of the loss of centrifugal force.

 

[looking4sophia]

If the Earth stopped spinning
then the Centrifugal Force FIELD would disappear.

If these Centrifugal Forces were gone, the Volume and Diameter of the Earth would shrink
because Internal Pressures would slightly increase the Density of the Mantel and Cores, imo.

Early Mathematical Models of our Gravity Field had to be adjusted for 'missing mass' which turned out to be underestimating the
extremely DENSE iron and other elements under the HyperPressures near the core.
These Compressive Pressures would Increase as the Centrifugal Forces Decreased if the Earth's spin slowed.

 

[Triple88a]

You bring up a pretty good point. Theres a good chance the surface pressure would increase as well.