Rheonomous

A mechanical system is rheonomous if its equations of constraints contain the time as an explicit variable.

[1][2] As shown at right, a simple pendulum is a system composed of a weight and a string.

Being inextensible, the string has a constant length.

The situation changes if the pivot point is moving, e.g. undergoing a simple harmonic motion where

The distance between the top end and the weight must stay the same.