A mass \(m\) is suspended from a spring of stiffness \(4000 \mathrm{~N} / \mathrm{m}\) and is subjected to a harmonic force having an amplitude of \(100 \mathrm{~N}\) and a frequency of \(5 \mathrm{~Hz}\). The amplitude of the forced motion of the mass is observed to be \(20 \mathrm{~mm}\). Find the value of \(m\).