I have the following problem: "A cannon of mass 5.80 × 103 kg is rigidly bolted to the earth so it can recoil only by a negligible amount. The cannon fires an 85.0 kg shell horizontally with an initial velocity of 551 m/s. Suppose the cannon is then unbolted from the earth, and no external force hinders its recoil. What would be the velocity of tan identical shell fired by this loose cannon? (Hint: In both cases assume that the burning gunpowder imparts the same kinetic energy to the system.)" My question is, is that assumption from the hint at the end unrealistic? I understand that the amount force the gunpowder exerts might be the same, but the kinetic energy would not, since in the first part the cannon absorbs whatever force from the gunpowder comes its way, thus there should be less kinetic energy in the first part than in the second part where the cannon is unbolted. Thus, is the assumption just used for the sake of solving the problem, and not actually a realistic one?