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