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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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closed as unclear what you're asking by Kyle Kanos, John Duffield, user36790, Bill N, Gert Nov 4 '15 at 21:49

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ 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
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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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