A mass of $m = 0.2$ kg suspended on a spring oscillates with frequency $f = 2$ Hz. Find the stiffness of the spring.
The frequency of oscillation of a spring pendulum: $f = \frac{1}{2\pi}\sqrt{\frac{k}{m}}$. Express the stiffness:
$$k = m (2\pi f)^2 = 0.2 \cdot (2\pi \cdot 2)^2 = 0.2 \cdot 16\pi^2 = 3.2\pi^2 \approx 31.6 \text{ N/m}$$
$k \approx 31.6$ N/m