**Ask a monkey a physics question thread**

[MadMojoMonkey]

So how does Newton explain the moon not crashing into Earth? It's inertia is what is keeping it going?

Why then (from either N or E's perspective) do some orbits decay? The thing is not going fast enough to overcome the pull of gravity I suspect?

Gravity is one form of energy. It's effect on orbits is not the only effect.

Consider the tides are caused by the moon (and the Earth's rotation under the seas, to a degree). There is energy transfer from the Moon's orbital energy into tidal energy. So the energy in the moon's orbit must not be constant.

There are other effects. Tidal heating is when the gravitation is not merely enough to pull water around, but the actual rock the body is made of. Many of Jupiter's moons have volcanic activity due to the constant squeezing and stretching of the moon like a stress ball. It heats the interior of the moon and keeps it molten.

Other effects serve to circularize orbits over time. Elliptical orbits will slowly become more and more round as gravitational energy is transformed back and forth into other forms.

[Poopadoop]

Gravity is one form of energy. It's effect on orbits is not the only effect.

Consider the tides are caused by the moon (and the Earth's rotation under the seas, to a degree). There is energy transfer from the Moon's orbital energy into tidal energy. So the energy in the moon's orbit must not be constant.

There are other effects. Tidal heating is when the gravitation is not merely enough to pull water around, but the actual rock the body is made of. Many of Jupiter's moons have volcanic activity due to the constant squeezing and stretching of the moon like a stress ball. It heats the interior of the moon and keeps it molten.

Other effects serve to circularize orbits over time. Elliptical orbits will slowly become more and more round as gravitational energy is transformed back and forth into other forms.

That's interesting but answers none of my questions.

Can Newtonian physics explain why one body will orbit another in a more or less stable way whereas another's orbit will decay into crash phase?

[MadMojoMonkey]

That's interesting but answers none of my questions.

Can Newtonian physics explain why one body will orbit another in a more or less stable way whereas another's orbit will decay into crash phase?

Yes, and it all has to do with the movement and concentration of energy from one form to another.

I described some forms of energy transfer, which would contribute to these different orbital behaviors. There are more avenues of energy transfer, but I'd have to know specifically what you're talking about to answer the mechanism involved.

In general, objects in orbits do not "decay into crash phase." The most prevalent decay phase is for an elliptical orbit to become more circular over time.

Not all decays involve the orbiting objects moving toward each other. The moon's orbital decay is taking it away from the Earth, not toward it.

[Poopadoop]

Not all decays involve the orbiting objects moving toward each other. The moon's orbital decay is taking it away from the Earth, not toward it.

So this leads into my question, just in an opposite form. What are the properties of a stable orbit that make it different from one that decays? Or do all orbit situations decay and if so, why?