The equation of motion can be expressed as $M(t)\ddot{q}(t) + D(t)\dot{q}(t) + K(t)q(t) = f(t)$ where $q(t)$ is the defection, $M(t)$, $D(t)$, and $K(t)$ are the mass, damping, and stiffness coefficients, respectively, and f(t) is the externally applied force.
The equation of a lumped circuit can be expressed as $di/dt + 10 log(i) - 20 = v(t)$ where $i$ is the current and $v(t)$ is external voltage source.
Are they linear or nonlinear, time-invariant or time-varying ? Why ?
I think they may not be the linear system since they have got the second-derivative on 1. and logarithm on 2. And both of them are time-invariant.
Not sure for my thought.