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Aug24-11, 04:03 AM
P: 950
Consider two particles performing simple harmonic motion. Like two masses hanging on two different vertical springs. If the masses go to the maximum height together and go to the lowest height together, then the two particles move in phase with each other. But if when one is at the bottom, the other is at the top, then they are out of phase with each other by half a period (or by 180 degress or by pi radians).