Let say a plane flights at 1km/s, shoots a 2kg projectile at 1km/s, so to the Earth, the projectile flights at 2km/s with 2*2*2/2 = 4MJ. The same gun when fixed on Earth, shoots the same 2kg projectile at 1km/s, achieves 2*1*1/2 = 1MJ Assume that the energy comes from burning propellant or electrical energy (railgun), how can the gun on the plane does more work than the same gun fixed on Earth? Is the additional energy from the plane? How can energy transfer from plane to the projectile? I.e. energy is transferred by force, and the pressure that pushes the projectile is the same in both cases. How can the projectile fired from a plane has higher energy?