Is Momentum Conserved?
Most recent answer: 10/15/2017
- Bob Berenz (age 63)
Miami, Fl, US
It's easy to lose kinetic energy without losing momentum. When two things that had been moving at different velocities collide and stick, the KE of their reltive motion is lost, but nothing happens to the total momentum. They're really different quantities, not even measured in the same units.
As for Newton's cradle, let's look at something even simpler, a pendulum. The momentum you see changes sign on each swing, just as for Newton's cradle. So that easily visible momentum isn't close to conserved. The reason is that the support string is transferring momentum to and from the Earth, where it's hard to notice.
Experiments and observations of everything from single particles to colliding black holes that confirm that momentum is conserved. In addition, the entire framework that makes sense of essentially every observation requires, by Noether's theorem, that momentum is conserved. Failure to notice that the balls are attached to strings is not sufficient grounds to throw out the framework.
(published on 10/15/2017)