# If earths rotation stoped, the moon would be released.

A friend insist that gravity is due to earths rotation and that if it stoped spinning the moon would sease to orbit and would be lauched in space!

Is that right?

What pourcentage of earth's gravity is caused by it's rotation?

I thought the gravity came from it's mass!

Thanks

Eric

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berkeman
Mentor
A friend insist that gravity is due to earths rotation and that if it stoped spinning the moon would sease to orbit and would be lauched in space!

Is that right?

What pourcentage of earth's gravity is caused by it's rotation?

I thought the gravity came from it's mass!

Thanks

Eric
Welcome to the PF.

As you and your friend take more physics and science classes, you will learn that gravity is due to mass, and has nothing to do with the spinning of the masses on their axes:

http://en.wikipedia.org/wiki/Newton's_law_of_universal_gravitation

.

Thanks! I owe you have the beer I just won...lol!

You are entirely right Eric; gravity comes from mass, not rotation.
Earth's rotation has a very very very small effect on the surface gravity (decreases it a little bit); but virtually no effect on the moon.

"...virtually no effect on the moon."

Other than the tidal dissipation making Moon recede just fast enough to be measured by laser ranging...
;-))

Phobos
Staff Emeritus
Gold Member
...and tidally locking the moon

DaveC426913
Gold Member
I think tidal locking of the Moon is due to the Moon's irregularity, not due to Earth's rotational irregularity.

sophiecentaur
Gold Member
I have a feeling that, but for a loss mechanism, like the tides, the irregularity of the Moon would just produce an undamped rotational oscillation (yawing) - not locking.
The tides, of course, are also the mechanism that is 'dragging' the Moon backwards and increasing the orbital radius at a very low rate.

[edit - owch, I meant Forwards]

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shashankac655
Is the moon spending any energy by revolving around the earth??? i heard that the moon is constantly losing it's angular momentum and is moving away from the earth and one day(over millions of years) break free of earth's gravity?

DaveC426913
Gold Member
Is the moon spending any energy by revolving around the earth??? i heard that the moon is constantly losing it's angular momentum and is moving away from the earth and one day(over millions of years) break free of earth's gravity?
Yes and no.

If both Moon and Earth were solid perfect spheres, and in a neighborless Solar System, no. In this ideal circumstance no energy is expended. They would happily revolve for eternity.

But both Moon and Earth are irregular ibodies, and Earth is not totally solid (oceans). They pull on each other imperfectly. This push and pull results in a slight acceleration of the Moon in its orbit. Faster orbit means higher orbit, so the Moon starts receding. But the Moon's recession will not continue forever, it will reach an equilibrium, and will not result in it escaping Earth's orbit. That would require a catastrophic event.

So, in reality, they are transferring energy back and forth - though even at that, the Earth-Moon system is not expending any energy.

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Yes and no.

But both Moon and Earth are irregular ibodies, and Earth is not totally solid (oceans). They pull on each other imperfectly. This push and pull results in a slight acceleration of the Moon in its orbit. Faster orbit means higher orbit, so the Moon starts receding. But the Moon's recession will not continue forever, it will reach an equilibrium, and will not result in it escaping Earth's orbit. That would require a catastrophic event.
Is this anything like the asteroid belt in our solar system, its pulled between the sun and jupiter and has found an equilibrium?

Janus
Staff Emeritus
Gold Member
Is the moon spending any energy by revolving around the earth??? i heard that the moon is constantly losing it's angular momentum and is moving away from the earth and one day(over millions of years) break free of earth's gravity?
The Moon is constantly gaining angular momentum.(As it loses orbital velocity, it gains even more in orbital radius.) The tidal coupling between the Earth and Moon is transferring angular momentum from Earth to Moon and the Earth's rotation slows in return.

The Moon won't break free of the Earth. After billions of years, the Moon and Earth would become tidally locked, and the Earth will rotate with the same period as the Moon orbits. When this happens, the mechanism that transfers angular momentum from Earth to Moon goes away and they reach a stable configuration( More or less, the Sun's tidal friction on the Earth will work against the Earth's rotation, in reaction, the Moon will start to transfer angular momentum to the Earth and will start to "reel in" towards the Earth.

More importantly, however, is the time scale involved. Before the Moon and Earth can even become tidally locked, our Sun will swell into a red giant, likely engulfing both Earth and Moon.

Yes and no..... So, in reality, they are transferring energy back and forth - though even at that, the Earth-Moon system is not expending any energy.

edit: oops, I think Janus just said that

moon is bound in its orbit due to earths gravity .It is revolving around earth since there is no external force to stop it ,so even earth stops rotation moon will continue its revolution until any external force stop it .

sophiecentaur
Gold Member
It must be realised that the scenario of the Earth's losing rotation is not going to happen. Any event that could cause that would have to be so precise and massive that the Moon would also be hugely affected.

DaveC426913
Gold Member
It must be realised that the scenario of the Earth's losing rotation is not going to happen. Any event that could cause that would have to be so precise and massive that the Moon would also be hugely affected.
What?

The Earth is slowing its rotation and will eventually be tidally locked with the Moon.

sophiecentaur
Gold Member
What?

The Earth is slowing its rotation and will eventually be tidally locked with the Moon.
Sorry. I meant completely lost. That couldn't happen as angular momentum of the system will be conserved.

A glance at Newtons's Law of Universal Gravitation will tell you that gravity is always a two-way street - just as the moon raises tides on earth, so earth must cause tides to sweep across the surface of the moon. These tides are not in water, of course, but in the solid rock of the lunar surface. In an amazing piece of planetary heavy lifting, the moons crust would have been distorted by up to 7 meters!
This giant tidal bulge sweeping across the moon had an interesting effect. As the moon turned beneath the Earth, the rock tide was dragged across its surface, but the rising of the tides isn't instantaneous; it takes time for the surface of the moon to respond to the pull of the earth. During that time, the moon will have rotated a bit, carrying the peak of the rock tide with it. The tidal bulge will therefore not be in perfect alignment with Earth, but slightly ahead of it. Earths gravity acts in the misshapen Moon in such a way that it tries to pull it back into sync; in other words, it works like a giant brake. Over time, this effect, known as Tidal Locking, gradually synchronizes the rotation rate of the moon with its with its orbital period, effectively meaning that the tidal bulge can remain in exactly the same place on the moons surface beneath the Earth and doesn't have to be swept around.
The moon is now almost, but not quite, tidally locked to Earth, which means that it takes one month to orbit the earth and one month to rotate around on its axis. So there's no dark side of the moon-the side we can't see gets plenty of sunlight, it's just a side that perpetually faces away from the Earth. The Earth-Moon system is in fact still evolving towards being perfectly tidally locked, and in one interesting consequence of this is that the Moon is gradually drifting further and further away from the Earth at a rate of just under 4 centimeters per year.

I hope this helps.

........ and in one interesting consequence of this is that the Moon is gradually drifting further and further away from the Earth at a rate of just under 4 centimeters per year.
Though I knew this was happening, it only just now occurred to me that the drift must be accelerating, even if only at the tiniest rate.

sophiecentaur
Gold Member
Though I knew this was happening, it only just now occurred to me that the drift must be accelerating, even if only at the tiniest rate.
What would be the reasoning behind that assertion? Are you referring to the 1/r potential?

What would be the reasoning behind that assertion? Are you referring to the 1/r potential?
Excuse my ignorance, I may know it as something else, but what is the 1/r potential?

As for my reasoning: At some point, perhaps when the moon breaks free, it will depart from us at a much faster rate. Unless the drift remains at a constant speed until it does break free, then it must be accelerating away from us even now, if only at a tiny rate of acceleration.

sophiecentaur
Gold Member
Gravitational potential follows a 1/r law(?).
Why do you suggest that the Moon can "break free"? That is not what is happening at all.

What?

The Earth is slowing its rotation and will eventually be tidally locked with the Moon.
I believe Sophie was refering back to the OP which implied a sudden, abrupt cessation of rotation.

Though I knew this was happening, it only just now occurred to me that the drift must be accelerating, even if only at the tiniest rate.
Even though the Moon is drifting away, Gravity has an infinite range, the Moon will not just drift off into space and loose its orbit with Earth.

Gravitational potential follows a 1/r law(?).
Why do you suggest that the Moon can "break free"? That is not what is happening at all.
I read the following from a http://en.wikipedia.org/wiki/Gravitational_potential" [Broken]

In classical mechanics, the gravitational potential at a location is equal to the work (energy transferred) per unit mass that is done by the force of gravity as an object moves to that location from a reference location. It is analogous to the electric potential with mass playing the role of charge. By convention, the gravitational potential is defined as zero infinitely far away from any mass. As a result it is negative elsewhere.

I think I understand it, but I could be wrong. I'm not sure I understand the "as an object moves" part. Is there a better way of wording it?

Why do I suggest the Moon can break free? If it's moving away from us at 4cm/year then at some point won't its own inertia will be stronger than the gravity which keeps it in orbit? Or will it remain in orbit, regardless of distance (at least until some other body pulls it from us)?

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