Would a mechanical system in a particular frame (when I say mechanical I mean pulleys, elastic strings, etc.) that has no potential energy (loaded springs, pulled back elastic, hanging weights, etc.) or kinetic energy be at zero energy state? I am aware of zero energy state in the quantum context, but that is not what I am referring to here. I am asking this because I am in the process of building a projectile launcher for a school project and it must be at "zero energy state prior to launching the projectile," (excluding the potential energy of a 5 lb weight that will power the entire launcher.)


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  • 2
    $\begingroup$ Uh...how are you defining "zero energy state" classically if not by having no potential or kinetic energy? $\endgroup$ – ACuriousMind Nov 4 '15 at 15:25
  • $\begingroup$ Well, no potential or kinetic energy in your local reference frame. There seems, perhaps, to be a real question in this, but it isn't coming out very well. $\endgroup$ – Jon Custer Nov 4 '15 at 16:51
  • $\begingroup$ A comet in a parabolic orbit about the sun has zero mechanical energy. That's a basic sophomore mechanics drill problem. $\endgroup$ – Bill N Nov 4 '15 at 21:00
  • $\begingroup$ My apologies for any lack of clarity. ACuriousMind, my question boils down to "What is zero energy state?" so I don't understand why I would need to define it. Jon Custer, the real question is, would a local frame without potential or kinetic energy be considered zero energy state? $\endgroup$ – dirk1212 Nov 5 '15 at 15:43

In classical mechanics, potential energy is only defined up to an arbitrary constant, and therefore total energy is only defined up to that arbitrary constant: in addition, the kinetic energy is reference-frame-dependent, and in the case of, say, $1/r^2$ force laws, it may not have a well-defined minimum.

For these reasons, "zero energy state" has no well-defined meaning, and saying that the system is "at a global minimum" of its potential energy (perhaps also saying that it is "stationary" or "in equilibrium" to clarify that it is stuck there) is more clear and well-defined.


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