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Did I missed something in angular momentum definition? Two identical bodies rotate around mass center. Now I invented anti-gravity and turning gravitational switch off. Those two bodies will move now in straight line with constant velocity and angular momentum conservation is compromised. Turning the gravitation off does not provide any external torque to the system. I also could not find any example when angular momentum is conserved and no internal forces (i.e. gravitation, Coulomb, tension) exist in the system. The definition of angular momentum does not said that existing of internal forces are necessary.


marked as duplicate by ACuriousMind, Bernhard, Carl Witthoft, ja72, Jim Aug 11 '14 at 18:37

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One does not need to switch "gravity off" to make a system like that. Replace gravity with a simple string, and cut the string. Physically that's completely equivalent to your problem, as far as I can see.

And while this may seem counterintuitive, the angular momentum in a system of a mass rotating at the end of a string is, indeed, conserved, when you cut the string and the mass flies away along the tangent of its circular trajectory!

The easiest way to see this is to look at the vector definition of angular momentum, which is the cross product of the radial vector with the momentum vector.


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