# Does the Moon's Gravity Attract Earth?

• lwymarie
In summary: Since the moon also has gravity, does it attract earth?Absolutely! The moon and Earth exert the same force on each other. This is why we have high tides when the moon is out, and as we're 70% (roughly) water ourselves, it must also effect us...
lwymarie
since the moon also has gravity, does it attract earth?

lwymarie said:
since the moon also has gravity, does it attract earth?
Absolutely! The moon and Earth exert the same force on each other.

thats why we have high tides when the moon is out, and as we're 70% (roughly) water ourselves, it must also effect us...

and why do planets orbit the sun but not bump into it?

Doc Al said:
Absolutely! The moon and Earth exert the same force on each other.

the same force? but Earth's gravity is greater!

hexhunter said:
thats why we have high tides when the moon is out,
The moon's tidal effect on the Earth is due to the variation of the moon's gravitational pull on the earth, which exerts a stretching force along the earth-moon line. Thus there are two tidal bulges: one on the side of the Earth nearest the moon, one on the other side of the earth. And thus, due to the Earth's rotation, approximately two high tides per day. (Whether the moon is "out" or not.)

and as we're 70% (roughly) water ourselves, it must also effect us...
First off, tidal forces affect everything: solid Earth as well as the oceans. Of course the oceans, being fluid, deform more easily. The tidal force on your body due to the moon would be ludicrously tiny.

lwymarie said:
the same force? but Earth's gravity is greater!
The gravitational field of the Earth is certainly greater than the moon's, but the gravitational force they exert on each other is the same! (This must be true from Newton's 3rd law.) Consider that the moon's smaller gravitational field acts on the much larger Earth will exert the same force as the Earth's larger field does on the smaller moon. The gravitational force between the Earth and moon is given by this formula:
$$F = G \frac{M_{earth} M_{moon}}{R^2}$$

Note that it doesn't matter which is which--two objects always pull on each other with the same force. (Of course, since the moon is much smaller than the earth, that same force will have a much greater effect on the moon than on the earth. The Earth barely budges, but the moon circles the earth!)

and why do planets orbit the sun but not bump into it?
Because they are moving sideways! If you could somehow stop the sideways motion of a planet, that planet would then fall into the sun.

lwymarie said:
the same force? but Earth's gravity is greater!

The force that acts on two bodies due to gravity is proportional to the product of both masses. The same magnitude of force acts on both bodies.

hexhunter said:
thats why we have high tides when the moon is out, and as we're 70% (roughly) water ourselves, it must also effect us...
It is believed that on full moons, the moon has an effect on the water in our bodies, causing more homicides and scuicides. This is where lunatic comes from luna=moon

Doc Al said:
...The tidal force on your body due to the moon would be ludicrously tiny.

Ludicrously tiny is right. If your car (~1000-2000 kg) is up on the rack for an oil change, and you (a 2 meter tall person) stand underneath it, your head 1 meter from the car, and your feet 3 meters away, quick "back-of-the-envelope" caluclations show that the car exerts a tidal force on your body about 100,000 times stronger than the tidal force exerted by the Moon.

Now there's an interesting factoid - thanks, tony.

Doc Al said:
Because they are moving sideways! If you could somehow stop the sideways motion of a planet, that planet would then fall into the sun.

Sorry I don't understand what 'sideway' means. Would you like to further explain please?

hexhunter said:
thats why we have high tides when the moon is out, and as we're 70% (roughly) water ourselves, it must also effect us...

does that mean that only 30% of you has to exert effort when you swim? jk

lwymarie said:
Sorry I don't understand what 'sideway' means. Would you like to further explain please?

$$\begin{picture}(150,150)(0,0) \put(30.,19){\circle*{200}} \put(130.,19){\circle*{5}} \put(130.,19){\vector(0,1){30}} \put(130.,19){\vector(-1,0){30}} \put(140.,39){v} \put(90.,0){F} \end{picture}$$

Basically, the planet is moving in the direction of the "v" vector and the star is pulling it in the direction of the "F" vector. Newton's first law says that an object in motion wants to continue motion in the same direction, so the planet wants to move forward. The force from the sun, however, is pulling it inward. In the next moment, it will have moved forward a bit, but it will also have moved in towards the star a bit, so its net motion will be forward and slightly to the left. It will continue to this behavior as time goes on and the total effect will be motion in an ellipse. The Earth's orbit is circular and that's a special case of an ellipse.

SpaceTiger said:
$$\begin{picture}(150,150)(0,0) \put(30.,19){\circle*{200}} \put(130.,19){\circle*{5}} \put(130.,19){\vector(0,1){30}} \put(130.,19){\vector(-1,0){30}} \put(140.,39){v} \put(90.,0){F} \end{picture}$$

Basically, the planet is moving in the direction of the "v" vector and the star is pulling it in the direction of the "F" vector. Newton's first law says that an object in motion wants to continue motion in the same direction, so the planet wants to move forward. The force from the sun, however, is pulling it inward. In the next moment, it will have moved forward a bit, but it will also have moved in towards the star a bit, so its net motion will be forward and slightly to the left. It will continue to this behavior as time goes on and the total effect will be motion in an ellipse. The Earth's orbit is circular and that's a special case of an ellipse.

so why some orbits are ellipses and some are circles? what is the principle behind?

lwymarie said:
so why some orbits are ellipses and some are circles? what is the principle behind?

The things that determine whether or not the orbit is a circle or ellipse are the magnitude and direction of the planet/moon's velocity (the "v" in the diagram above) and the distance between it and the more massive body. There are some values for which it will move in a circle, some in an ellipse, and some in a hyperbola! That last one would mean that the little object is escaping the gravity of the bigger one.

lwymarie said:
so why some orbits are ellipses and some are circles? what is the principle behind?

Circular orbits don't really exist except on paper. (Same for parabolic orbits.) An object with an eccentricity of 0.00 could be said to be in a circular orbit. But if you looked further to the right of the decimal point it's doubtful that the object would have an eccentricity of 0.00000000.

The classical way to describe how an orbit works is to picture a person throwing a ball in a horizontal direction. If he throws it soft, it falls to the ground and its path is an arc. If he thows it harder, it goes farther before falling to the ground as the arc is shallower. If he throws it so hard that the arc of its drop matches the curvature of the Earth, then it never gets any closer to the Earth. For every foot it drops, the Earth curves off 1 foot and the ball is no closer to the ground. It will travel completely around the world in a circular orbit. (ignore air resistance, hills, mountains, etc.)

If he throws it a little harder than the speed needed for its falling arc to match the curvature of the Earth, than the ball would actually rise. But just like throwing a ball straight up, it would slow down, reach its high point and drop back down again. Only in this case, the lowest it could drop would be the height from which you released it. It would be in an elliptical orbit.

tony873004 said:
Circular orbits don't really exist except on paper.

This is true, but for these purposes, I think we can call the Earth's orbit circular.

lwymarie said:
so why some orbits are ellipses and some are circles? what is the principle behind?

It's hard to maintain a perfectly circular orbit...there are many gravitational nudges out there. (lot of other stars, planets, asteroids, etc. out there...each with their own gravitational influence on the system)

lwymarie said:
since the moon also has gravity, does it attract earth?

Everything with mass also has gravity.

YOUR gravitational field pulls on the Earth just as the Earth pulls on you. (As you might guess, the Earth wins that contest since it has so much more mass. More mass means more of a gravitational field.)

yomamma said:
It is believed that on full moons, the moon has an effect on the water in our bodies, causing more homicides and scuicides. This is where lunatic comes from luna=moon

The effect exists, the cause does not. The believed cause is that more light makes it easier to carry out outdoor activities.

lwymarie said:
so why some orbits are ellipses and some are circles? what is the principle behind?

All two body system orbits are ellipses and all circles are a special form of ellipse in which the minor and major axis are equal in length.

The general equation of an ellipse, in a coordinate system chosen such that no transformation in the x or y-axis is necessary, is $$ax^2+by^2=c$$. A circle is an ellipse in which a=b, which can always be transformed into $$x^2+y^2=c/a$$, in a coordinate system chosen such that the center of the circle is the intersection of the x and y axis. (x+d) and (y+e) could replace the x and y terms to obtain a general system in any cartesian coordinate system, but it would look messier.

In a system where one object is very massive relative to the other, the smaller object will have a nearly circular orbit around the larger object.

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Doc Al said:
The gravitational field of the Earth is certainly greater than the moon's, but the gravitational force they exert on each other is the same! (This must be true from Newton's 3rd law.) Consider that the moon's smaller gravitational field acts on the much larger Earth will exert the same force as the Earth's larger field does on the smaller moon. The gravitational force between the Earth and moon is given by this formula:
$$F = G \frac{M_{earth} M_{moon}}{R^2}$$

Note that it doesn't matter which is which--two objects always pull on each other with the same force. (Of course, since the moon is much smaller than the earth, that same force will have a much greater effect on the moon than on the earth. The Earth barely budges, but the moon circles the earth!)

Because they are moving sideways! If you could somehow stop the sideways motion of a planet, that planet would then fall into the sun.

in velikovsky's worlds in collision, he describes the collisions of venus, Mars and Earth within the last 8 to 15,000 years. Isn't it possible that such a collision could happen with the sun, assuming of course that any of the body in question withstood the temperatures?

yomamma said:
It is believed that on full moons, the moon has an effect on the water in our bodies, causing more homicides and scuicides. This is where lunatic comes from luna=moon
'Lunatic' better describes the witchcraft-fearing fools that believe such susperstitious nonsense. Tidal/gravitational forces act the same on the 70% of us that's water as they do on the 30% of us that's made of non-water.

x8jason8x said:
in velikovsky's worlds in collision, he describes the collisions of venus, Mars and Earth within the last 8 to 15,000 years. Isn't it possible that such a collision could happen with the sun, assuming of course that any of the body in question withstood the temperatures?

I highly doubt it.

Simply put:

1. In a stable orbit, angular momentum is balanced against graviational pull towards the center.

2. Your natural intution that angular momentum will in time give way to gravitational pull is based on the fact that in any Earth based analogy (such as a spiral coin drop in a museum or anything in the air) friction does errode angular momentum. But, in a vacuum, there are virtually no frictional effects, and the planets have, over time, cleared away most of the dust that could cause friction in their paths.

3. There is considerable evidence that the planets are billions of years old.

4. The planets could not have stayed in place for billions of years if they were not in stable orbits.

5. Projections of their orbits show that they are basically stable, even though a many body system has slight inherent chaos which tends towards attractors which are basically stable.

6. We are talking millions or billions of years.

See e.g. http://www.umich.edu/~urecord/9899/Jun07_99/6.htm

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I know this is a bit off topic with the Earth and moon, but it has to do with everything with mass having gravity. Up in space astronauts float because they're far enough away from any planets or the moon that no gravity is keeping them to the floor, right? Well the astronauts, and the shuttle they're in, and everything near them has gravity, right? So why don't the things with large masses, like the space shuttle, attract other things like the astronauts and keep them to the floor? They both have gravitational pull so you wouldn't think they'd just float around. Or why doesn't the gravitational pull of the astronauts attract a piece of paper or another astronaut, since they don't have Earth's gravity to make a greater influence?

Another question, if the Earth's gravitational pull in space is enough to keep the moon or a space shuttle in orbit, why isn't it enough to pull the astronauts towards it and keep them to the floor (or ceiling or whatever's closest) instead of letting them float around?

Newton's first law states that a body at rest will remain at rest and a body in motion will continue in motion with constant speed in a straight line, as long as no unbalanced force acts on it.

So does that mean that motion is acausal? Do bodies stay in motion for no reason at all? If a particle at rest is caused to move in a certain direction, what keeps it moving in the same direction after the initial force is taken away?

I know this is a bit off topic with the Earth and moon, but it has to do with everything with mass having gravity. Up in space astronauts float because they're far enough away from any planets or the moon that no gravity is keeping them to the floor, right? Well the astronauts, and the shuttle they're in, and everything near them has gravity, right? So why don't the things with large masses, like the space shuttle, attract other things like the astronauts and keep them to the floor? They both have gravitational pull so you wouldn't think they'd just float around. Or why doesn't the gravitational pull of the astronauts attract a piece of paper or another astronaut, since they don't have Earth's gravity to make a greater influence?
Yes, everything does attract everything else in a space shuttle, this is why the term "microgravity" is used to describe this situation. The reason things held to the floor are twofold; The items are in the shuttle so its mass surrounds them and pulls in all directions and the gravity is very, very small. The orbiter has a mass of 75,000 kg. if you were standing on its hull, the closest you could get to its center would be about 3 meters. Acceleration due to gravity is equal to
$$A = \frac{GM}{d^2}$$
This works out to .000000556 m/s. This means that a dropped object would take 3 min to fall 1 cm.

Escape velocity would be 0.182 cm/s, meaning if the object was moving faster than this in any direction other than directly towards the shuttle, it would fly of into space.
Another question, if the Earth's gravitational pull in space is enough to keep the moon or a space shuttle in orbit, why isn't it enough to pull the astronauts towards it and keep them to the floor (or ceiling or whatever's closest) instead of letting them float around?

The astronauts and shuttle are both in freefall, meaning that the Earth's gravity pulls on both equally. You'd get the same effect if you were in an elevator and the cable snapped, you'd float around in the elevator in a weightless state (At least until the you hit the bottom of the shaft).

ohwilleke said:
The effect exists, the cause does not. The believed cause is that more light makes it easier to carry out outdoor activities.
Personally, I think it's more a skewed perception thing. How many times have you seen a crime committed and thought it was significant that it happened under a waning gibbous Moon?

Starship said:
So does that mean that motion is acausal? Do bodies stay in motion for no reason at all? If a particle at rest is caused to move in a certain direction, what keeps it moving in the same direction after the initial force is taken away?

The "why" of Newton's Laws is answered by Einstein's equations and Quantum Mechanics. The "why" of those theories may be explained by string theory. At some point, however, you're going to be stuck with a "why" without an answer. It's just the way things are.

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ohwilleke said:
The effect exists, the cause does not. The believed cause is that more light makes it easier to carry out outdoor activities.

I certainly agree that the moon does not cause abnormal human behavior through some physical mechanism.

But furthermore, I have yet to see any demonstration that there is a statistically significant increase in abnormal behavior associated with the phase of the moon. Seems to be an urban legend. But, if y'all want, please continue that discussion in the S&D forum.

x8jason8x said:
in velikovsky's worlds in collision,...

Velikovsky was a bit of a crackpot. So, let's not muddle this particular topic with his ideas. But you can discuss Velikovsky's ideas in a new topic if you want. Thanks.

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Phobos said:
Velikovsky was a bit of a crackpot. So, let's not muddle this particular topic with his ideas. But you can discuss Velikovsky's ideas in a new topic if you want. Thanks.

at the risk of mincing, Einstein was also called a crackpot...I tend not to dismiss anything as irrelevant until it's proved absolutely wrong. I will however refrain from "muddling".

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