Conservation of momentum

Hal Finney (
Sat, 30 Nov 1996 09:06:00 -0800

Conservation of momentum is a very well established physical law.
Conservation of energy can be a little trickier: you can have various
forms of energy, kinetic, chemical, heat, potential. It can be hard to
account for where the energy is in a system and therefore hard to tell
if a system is really conserving energy. Look at all the controversy
over whether the cold fusion experiments are producing more (non-nuclear)
energy than they are consuming.

But conservation of momentum is a lot easier to understand. Unless you
are dealing with relativistic velocities, or really overwhelmingly huge
energy densities, the momentum is just a matter of multiplying each moving
mass by its velocity, and adding all those up.

Conservation of momentum and the other laws of physics are the basis for
our entire technological civilization. If these laws were not reliable
bridges would not stand, rockets would not make it to the moon, machinery
would not operate. Centuries of careful testing and measurements have
only confirmed the accuracy of momentum conservation in all physical
systems. This is not just approximate conservation, this is perfect
conservation to the finest limits of accuracy available.

The notion that you are going to be able to come up with a
mechanical system which arranges moving weights "just so" and leads to
non-conservation of momentum is just so terribly unlikely as not to be
worth seriously considering.