$6\mathrm{.}98$ Consider a two$-$degree$-$of$-$freedom system with the equations of motion $[$m$]$vec$($x$)^()+[$k$]$vec$($x$)=$vec$($f$)($t$)$

with

$[$m$]=[[1,0],[0,4]],[$k$]=[[8,-2],[-2,2]],$ and $,$vec$($f$)($t$)=\{[$f$\_(1)($t$)],[$f$\_(2)($t$)]\}$

FIGURE $6\mathrm{.}41$ Bicycle on a road with step change in elevation.