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[Poopadoop]

Conservation of angular momentum.

In an isolated system, the angular momentum of the system is a constant.

Uh, yeah.

Now how about in English?

[MadMojoMonkey]

Uh, yeah.

Now how about in English?

Sorry.

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"In an isolated system,"
I'm talking about a spinning thing (or group of things) that is not being torqued or pushed by anything

"the angular momentum of the system"
The total amount of spinning-ness of the thing

"remains constant"
does not change.

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For clarity of nomenclature, spinning things don't fly straighter than non-spinning things, they maintain stability while flying just as straight as an unstable thing.
(or I've misunderstood your question.)

It's the rotational equivalent to Netwon's First Law
"In an inertial reference frame, an object in motion will remain in motion at a constant velocity if and only if the vector sum of forces acting on the object is 0 N."
(Note that 0 is a 0 vector, for the math nerds who care about that stuff.)

In other words:
Stuff changes how it moves only when it's pushed, but pushes which cancel each other out don't change how objects move.
Therefore, we can deduce if something has been pushed by seeing if it changes how it's moving. If it changes, it was pushed.

A nearly identical rule holds for spinning things. If they change, then they must have been pushed. If nothing is pushing (or all pushes are cancelling out), then it will not change.

Spinning things have a non-0 rate of angular momentum (actively spinning inertial mass). Since it is spinning, it will not change without a push... but now I have to clarify that a spinning push is a torque. So spinning things only change their spinning if they are torqued.

Spinning things have an axis of rotation, which partly defines their spinning. If the orientation of that axis of rotation changes, that requires a torque to have changed it. In general, the friction on a spinning object is such that it slows the spinning w/o changing the orientation of the spinning axis.

So, like, a frisby maintains stability because it is spinning and that spinning has an axis of rotation. In order to change the axis of rotation, a torque must be applied to the frisby. However, the air resistance acting on the frisby supplying a torque is generally pretty balanced around the edge of the frisby, putting roughly equal forces of friction all around the frisby. These tend to slow the rotation w/o changing the orientation of the rotation.

It occurs to me that a frisby always rolls over to one side and that since it is moving forward as it spins, the air velocity across the advancing edge of the frisby will not be the same as the friction along the retreating edge. This could cause a net torque which serves to drive the roll-over of the frisby.

[OngBonga]

Uh, yeah.

Now how about in English?

I'll give it a bash.

Take the earth-moon system, and simplify it to say that the moon is moving perpendicular to earth, while earth's gravity acts to draw the moon towards earth. If the moon was not moving perpendicular to earth, it would fall directly towards earth. But because it has linear velocity, then it moves in a circle around earth (I emphasise, very simplified).

That's conservation of energy.

Conservation of angular momentum is basically the same, only applied to a rotating frame of reference. If you imagine a spinning top, if you give it a push to try and make it fall over, it won't, because the part you made fall towards the direction of gravity has moved and is now going against gravity, before reacing its apex and going back down with gravity, etc.

Conservation of angular momentum is basically conservation of energy, just more complex because of the nature of motion.