The discussion centers on the mechanics of movement in isolated systems, specifically addressing why rockets and guns can move despite internal forces seemingly canceling each other out. Participants clarify that while internal forces do cancel, the systems in question (rocket + fuel, gun + bullet) are not isolated when considering the expelled mass. The conservation of momentum principle dictates that the center of mass remains unchanged unless external forces act on the system. Thus, the apparent contradiction arises from a misunderstanding of system boundaries and the nature of internal versus external forces.
PREREQUISITESStudents of physics, educators teaching mechanics, engineers involved in design and analysis of propulsion systems, and anyone interested in understanding the principles of motion and force interactions in isolated systems.
Depends on how you define the objects. You can consider Münchausen as arm & body to have two objects as well. Then analyze the forces between them to see what is actually different compared to gun & bullet.Kenan said:If forces will be acting on the same object then there will be no motion,but in gun or rocket case forces are not acting on the same object,no?
Doc Al said:OK.An object will accelerate (begin to move) if there is an unbalanced force acting on it. It the case of the bullet fired from a gun, there is an unbalanced force acting on the bullet as it begins moving.His hair won't move since it is attached to his head! (If he really pulls it, it could tear off though.) And his body won't move because whatever force he pulls up with (on his hair) is balanced by the equal and opposite force that the hair pulls down with.
Always ask yourself: Is there a net force on an object? If so, it will accelerate. If not, it won't.
Kenan said:Ok I think I understand you,in the Münhausen example net force is zero because forces are acting on the same object.In the rocket/gun example both are moving because net force is not zero - forces are acting on different objects.Am I right now?
Close enough.Kenan said:Ok I think I understand you,in the Münhausen example net force is zero because forces are acting on the same object.In the rocket/gun example both are moving because net force is not zero - forces are acting on different objects.Am I right now?
Yes. The net force on each 'piece' of the system is zero in the first example and non-zero in the rocket/gun example.Kenan said:Ok I think I understand you,in the Münhausen example net force is zero because forces are acting on the same object.In the rocket/gun example both are moving because net force is not zero - forces are acting on different objects.Am I right now?