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Phases of Harmonic Motion

In-phase: when two oscillating objects (like a pendulum) or waves are in the same point in their cycles. Two full moons are in-phase; two crests of a wave are in-phase.

Out-of-phase: when two oscillating objects or waves are in different points in their cycles. A full moon and a half-moon are out-of-phase; a crest and a trough (bottom) are out-of-phase.

If two waves are out-of-phase we need to know by how much. We define one complete cycle as 360° (or 6.28 radians). Then 1/2 cycle out-of-phase would be 180° (or 3.14 radians) out-of phase. 1/4 cycle out-of-phase is 90° (or 1.57 radians) out-of-phase. If two points are 90° out-of-phase, then the first one ise +90° ahead or the second one is -90° behind.

Two points or pendulums could be more than 360° out-of-phase depending on when they started. If a wave is 2 cycles ahead, then it is 720° ahead (or 12.36 radians), etc.