What does it mean to be financially responsible? It's a complex question with a complex answer, but at its core is a simple truth: To be financially responsible, you need to live within your means. And to live within your means, you must spend less than you make.
longitudinal axis. A straight line running fore and aft through the center of gravity. It is horizontal when the airplane is in the rigging position. The aircraft rolls about this axis. See axes.
In a longitudinal wave, a rest axis would apply only if we are considering a two-dimensional wave. There would be lines, perpendicular to the direction of travel of the wave, that would consist of particles that are undisturbed at a given moment. I don't see that the term would apply to a one-dimensional travelling longitudinal wave.