A mechanical system is rheonomous if its equations of constraints contain the time as an explicit variable.^[1][2] Such constraints are called rheonomic constraints. The opposite of rheonomous scleronomous.^[1][2]

As shown at right, a simple pendulum is a system composed of a weight and a string. The string is attached at the top end to a pivot and at the bottom end to a weight. Being inextensible, the string has a constant length. Therefore, this system is scleronomous; it obeys the scleronomic constraint

${\displaystyle {\sqrt {x^{2}+y^{2}}}-L=0\,\!}$,

where ${\displaystyle (x,\ y)\,\!}$ is the position of the weight and ${\displaystyle L\,\!}$ the length of the string.

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

${\displaystyle x_{t}=x_{0}\cos \omega t\,\!}$,

where ${\displaystyle x_{0}\,\!}$ is the amplitude, ${\displaystyle \omega \,\!}$ the angular frequency, and ${\displaystyle t\,\!}$ time.

Although the top end of the string is not fixed, the length of this inextensible string is still a constant. The distance between the top end and the weight must stay the same. Therefore, this system is rheonomous; it obeys the rheonomic constraint

${\displaystyle {\sqrt {(x-x_{0}\cos \omega t)^{2}+y^{2}}}-L=0\,\!}$.

• Lagrangian mechanics
• Holonomic constraints