# Why Hasn't the Moon Crashed into the Earth?

1. Dec 1, 2012

### Inductor

please don't mention the word centripetal or use the (misguided) rock-on-a-string analogy.
clue: there is no "balance of forces"

in a similar vein - please answer the following orbital skills question ... were the planet Saturn to stop rotating about it's axis - what would happen (if anything) to the Rings of Saturn? Would they fling out into deep-space? Would they glom into a big ball and catch fire? Would they come crashing down and burn up (ionize) in the Saturnian atmosphere? What exactly would happen to them?

2. Dec 1, 2012

### Staff: Mentor

What's misguided about it? And what do you have against the word centripetal? (It's kind of important.)
That's true. F = ma.

3. Dec 1, 2012

### ImaLooser

If Sat stopped rotating its gravity would be same. The rings would be little affected.

4. Dec 1, 2012

### Bandersnatch

The Moon as going sideways fast enough to always keep missing the Earth as it falls towards it.

Also, what DocAl said. What's wrong with centripetal force?

You're probably thinking of the equation:
Fc=Fg
and reason that this is a bunch of baloney, because not only gravity is the only force acting(hence, why for is the other one there?), but if the two forces were equal, then Newton's first law of motion would apply(the Moon would have to either stand still or move in a straight line.

The trick is, this equation is not an equation of motion.
It merely says that for a body(e.g.the Moon) to keep moving in circles, the force acting on that body(here Fg) must have a very specific magnitude, which is described as Fc.

Fc is not a real force, it's just a mathematically derived value that you need your force to be for your body to have it's velocity vector constantly changing its direction, but not its magnitude(aka moving in circles).

5. Dec 3, 2012

### BobG

This would depend on what stopped Saturn from spinning. Conservation of momentum means the angular momentum from Saturn's rotation has to go somewhere. Whatever stopped Saturn's rotation would probably have a drastic affect on the rings, as well.

And if Saturn had never been spinning in the first place, then you wouldn't have rings, either. The rotation of the planetary disk created both.

But, given the unrealistic conditions that Saturn violated the known laws of phsyics and stopped spinning for no reason at all, the rings wouldn't be affected since they're motion is independent of the planet's rotation once the rings after the rings have been formed.

Or, the change could happen gradually, as with the Earth and Moon. Because of tidal forces, the Earth's rotation gradually slows and the Moon's orbit gradually increases in size. This wouldn't stop the Earth's rotation, however. Once the Moon's orbit and the Earth's rotation reached the point where the same face of the Earth always faced the Moon, the Earth's rotation would stabilize.

And, it would be hard for this to happen to Saturn. There's more than one moon, plus you have the rings. In other words, the forces are pulling from many directions. Whatever the imbalance in tidal forces, it would seem like it would have to be much smaller than the imbalance created by the Earth's Moon.

Last edited: Dec 3, 2012
6. Dec 3, 2012

### Whovian

I agree completely. The moon's orbit's elliptical, but otherwise, once you know a little about Newtonian gravitation, the Rock-on-a-string Analogy is pretty much perfect.

And maybe it's centrifugal, not centripetal, you want us to avoid?

7. Dec 3, 2012

### Staff: Mentor

Um, let's see. Why is it you want us to avoid using standard explanations that work well for the past several hundred years? Do you have a source of some new revelation?

You do understand that Science works by repeatedly testing ideas (hypotheses) and adding new observations that either support or reject existing ideas. Do you have some new obserations?

8. Dec 4, 2012

### goldsax

if we are talking about moon and earth. the moon was constructed of the accretion of fragments after a giant collision of the embryo earth with another giant object.
so if the moon was going to fall towards the earth it would not have time to form in the first place.
as to the present state of affairs, the moon is drifting out by about 3cm a year