A $2\mathrm{.}0-$kg mass is attached to one end of a spring with a spring constant of $100$N$//$m 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 $2\mathrm{.}0-$kg mass has a speed of $0\mathrm{.}36$m$//$s$.$ The

mechanical energy that has been lost to this instant is: