A pulley \(250 \mathrm{~mm}\) in diameter drives a second pulley \(1000 \mathrm{~mm}\) in diameter by means of a belt. The moment of inertia of the driven pulley is \(0.2 \mathrm{~kg}-\mathrm{m}^{2}\). The belt connecting these pulleys is represented by two springs, each of stiffness \(k\). For what value of \(k\) will the natural frequency be \(6 \mathrm{~Hz}\) ?