3 ms·
Newton’s third law is just the conservation of momentum... and lots of interesting systems don’t conserve momentum!
by bodhiandphysics 5y ago
Newton’s third law is just the conservation of momentum... and lots of interesting systems don’t conserve momentum!
- nextaccountic 5y agoBut closed systems?
- bodhiandphysics 5y agoSure! A ball in a box is a closed system the doesn’t conserve momentum.
- nextaccountic 5y agoHow so? Consider that the evolution of the system will be the same regardless of where the box is located, and as such the system has translational invariance. Per Noether's theorem it must conserve linear momentum.
- bodhiandphysics 5y agoI meant the infinitely massive box of physicists dreams. Translational symmetry is broken by the boundaries of the world
- nextaccountic 5y agoYou mean, a box that is immovable because it has infinite mass? Hmmm I don't get it. What boundaries, the box boundaries? How does it break translational symmetry? Anyway in this case I think at least elastic collisions will still conserve momentum, eg this https://en.wikipedia.org/wiki/Elastic_collision#/media/File:Translational_motion.gif https://en.wikipedia.org/wiki/Elastic_collision#/media/File:... conserves momentum even if the box has infinite mass.