# Gravity of moon

1. Apr 30, 2005

### lwymarie

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

2. Apr 30, 2005

### Staff: Mentor

Absolutely! The moon and earth exert the same force on each other.

3. Apr 30, 2005

### hexhunter

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

4. Apr 30, 2005

### lwymarie

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

5. Apr 30, 2005

### lwymarie

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

6. Apr 30, 2005

### Staff: Mentor

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.)

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.

7. Apr 30, 2005

### Staff: Mentor

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.

8. Apr 30, 2005

### Janus

Staff Emeritus
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.

9. Apr 30, 2005

### tony873004

10. Apr 30, 2005

### JamesU

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

11. Apr 30, 2005

### tony873004

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.

12. Apr 30, 2005

### Staff: Mentor

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

13. May 2, 2005

### lwymarie

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

14. May 2, 2005

### aychamo

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

15. May 2, 2005

### SpaceTiger

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

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.

16. May 2, 2005

### lwymarie

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

17. May 2, 2005

### SpaceTiger

Staff Emeritus
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.

18. May 2, 2005

### tony873004

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.

19. May 2, 2005

### SpaceTiger

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

20. May 2, 2005

### Phobos

Staff Emeritus
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)