15-2.

We are given the mass $m$, amplitude $A$, and period $T$ of a body undergoing Simple Harmonic Motion. Newton's Second Law tells us that

$$F=-kx,$$

so that $F$ is maximum when $x$ is maximum, that is, when we have $x=A$. Thus,

$$\begin{eqnarray*} F_{max}&=&kA\\ &=&(\omega^2m)A, \end{eqnarray*}$$

since we have a spring system. We can obtain $\omega$ from $T$ by

$$\omega=2\pi f=\frac{2\pi}{T}.$$

This yeilds a value for $k$ of

$$k=(2\pi f)^2m=118\;\textrm{N/m},$$

and a value of the maximum force of $10\;{\textrm N}$.