= Rigid-body translation
A <rigid body> undergoes translation when every point has the same <displacement> at each time, so its orientation stays fixed. For a freely moving system of particles coupled only by internal springs, the common <displacement> is a zero-frequency <normal mode>: internal spring lengths and <potential energy> are unchanged. Its mode coordinate is $A+Bt$, representing constant <centre-of-mass velocity>, rather than an oscillation at nonzero <frequency>.
Back to article page
