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Physics/Conservation of Angular Momentum


Suppose that in a closed system, a ball is rotating around a center, both connected by a string. When the centripetal force disappears (i.e. if the string is broken), the ball will be released and travel in linear direction with tangential velocity. What has happened to the angular momentum in this situation? Initially, there is non-zero angular momentum since the ball is rotating. Then, the ball is travelling linearly since the centripetal force has disappeared and no angular momentum remains. Since there is no torque acting on the system, we can say that this system is a closed system. How can angular momentum be conserved in this condition?

No, you're forgetting that things which have momentum "p" relative to a center-of-line distance r from the point about which you are considering (the center of rotation, in this case), have an angular momentum of p*r for the system.  It's still conserved.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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