A $2\mathrm{.}0-$kg mass is attached to one end of a spring with a spring constant of $\backslash (100\backslash$mathrm$\{$~N$\}/\backslash$mathrm$\{$m$\}\backslash )$ and a $4\mathrm{.}0-$kg mass is attached to the other end. The masses are placed on a horizontal surface and the spring is compressed $10$ cm $.$ The spring is then released with the masses at rest and the masses oscillate. When the spring has its equilibrium length for the first time the $\backslash (2\mathrm{.}0-\backslash$mathrm$\{$kg$\}\backslash )$ mass has a speed of $\backslash (0\mathrm{.}36\backslash$mathrm$\{$~m$\}/\backslash$mathrm$\{$s$\}\backslash ).$ The mechanical energy that has been lost to this instant is: