Torque and Newton's Laws
Most recent answer: 01/08/2014
- Nathan (age 23)
Boston, MA, USA
To boil your question down a little further, what it's really getting at is whether Newton's Third Law (momentum conservation) implies the conservation of angular momentum. Unless angular momentum were to be conserved, you could get angular acceleration without any external forces.
It turns out that momentum conservation is not sufficient. Say you had two objects exerting forces on each other. Newton's Third Law says those forces are equally strong and point in opposite directions. Unless those directions are along the line connecting the centers of the two objects, the two torques (about the center of mass) won't be zero and they won't cancel. But the "along the line connecting the centers " rule is not one of Newton's three laws. So it's an extra law, a particular case of a general additional law, the conservation of angular momentum.
Mike W.
(published on 01/08/2014)