A system consists of a \(4.0-\mathrm{kg}\) cart and a \(1.0-\mathrm{kg}\) cart attached to each other by a compressed spring. Initially, the system is at rest on a low-friction track. When the spring is released, an explosive separation occurs at the expense of the internal energy of the compressed spring. If the change in the spring's internal energy during the separation is \(1.0 \mathrm{~kJ}\), what is the speed of each cart right after the separation?