For an undamped simple harmonic oscillator, which quantity stays constant throughout the motion?

1. An undamped oscillator has no dissipative forces, so no mechanical energy leaves the system. 2. Energy is traded back and forth between kinetic energy and elastic potential energy as the motion proceeds. 3. Their sum, written (1/2) m v squared + (1/2) k x squared, therefore stays constant. 4. Each part alone swings between zero and the full total, so neither one on its own is conserved. _Source: OpenStax College Physics (CC BY 4.0), Ch 16 "Oscillatory Motion and Waves", section 16.5 Energy and the Simple Harmonic Oscillator_