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Q & A: is momentum conserved?

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Most recent answer: 10/15/2017
Q:
Per Newton's Laws, momentum is a conserved quantity and yet both ironically and factually, a Newton's cradle clearly demonstrates momentum is lost with every collision. In every collision, kinetic energy transforms into other forms. And when ke diminishes, momentum must as well. A physicist has admitted this to me and yet, he still insists momentum is a conserved quantity. Why are so many physicists unwilling to admit they may have been fed information that might not be as valid as assumed? Stated in a far different way, physicists are willing to entertain such things as time travel and wormholes but completiely rule out that the scientists of distant past erred in some way and that some of those errors still exist today? This is a serious question.
- Bob Berenz (age 63)
Miami, Fl, US
A:

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.

Mike W. 


(published on 10/15/2017)

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