$2\mathrm{.}13.$ For the system shown in Figure P$2\mathrm{.}13,$ the displacement of $M_1$ relative to $M_2$ is

$x_1.$ The spring force is zero when $x_1=x_2=0.$ Draw free$-$body diagrams and write the

modeling equations.

Figure P$2\mathrm{.}13$