Energy Transformation for a Dart

In that case of the dart being launched from the spring gun and the just forces doing work upon the dart are internal forces. Initially, the dart is being acted upon through a spring force in sequence to be projected from the dart gun. The coils of the springs are initially compressed and upon pulling the trigger and the springs return to their equilibrium position although pushing the dart out of the dart gun. A dart then becomes a projectile (supposing there is negligible air resistance); the just force doing work upon the dart during its flight through the air is gravity.