# Gravity of the Earth decreases if it stops spinning?

• B
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.

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phinds
Gold Member
2019 Award
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?

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 wouldnt that contribute to the force of attraction towards the center of the earth the fact that its spinning ?

phinds
Gold Member
2019 Award
The mass effects gravity but when you spin an object it creates a force going towards the center cause of centripetal force. So wouldnt 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.

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 dont 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 dont think there the same but dont 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
Gold Member
2019 Award
I dont 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 dont think there the same but dont 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
Staff Emeritus
I dont 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 dont think there the same but dont 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.

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.

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
Staff Emeritus
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.

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
Staff Emeritus
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.

aiop
tony873004
Gold Member
So wouldnt 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.

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
Gold Member
2019 Award
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
Gold Member
2019 Award
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?

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!

aiop
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 ! :)

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!

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