Imagine I have a rocket with a certain amount of energy stored as chemical energy, lets say its 10Js, that exhausts itself after 5 seconds. If I attach this rocket to a (relative to an observers frame) stationary cart in such a way that it pushes the cart, it will add 10J of kinetic energy to the cart system. But if I attach this rocket to a moving cart, it will still burn for 5 seconds, and because this new cart is moving that means it will apply an equal force for a greater amount of distance. Thus, it should do more work and add more kinetic energy. But didn't it only start with 10J of chemical energy? Or instead of a rocket we could imagine an ideal engine which extract a certain amount of kinetic energy from its environment: A fixed amount of joules. If this engine is moving then it will extract the same amount of energy as heat, but won't it apply an equal force over a now greater distance?