An undamped Single Degree of Freedom (SDOF) system follows the differential equation:
\[
m \ddot{x} + kx = 0
\]
The general solution to this equation represents harmonic motion with angular natural frequency $\omega_n$ given by:
\[
\omega_n = \sqrt{\frac{k}{m}}
\]
Where:
$k$ = stiffness of the spring (N/m)
$m$ = mass of the system (kg)
Thus, the natural frequency of vibration (in rad/s) is $\omega_n = \sqrt{k/m}$.