![]() This means that the momentum stays constant, but you have to consider not only the momentum of the fragments but also the momentum of the shockwave. In the real explosion (so I'm not talking about the simple model I wrote above) there are internal forces between the shockwave made of compressed gas and each fragment. ![]() There will be some strong repulsion forces, but all of these are internal forces, so the total momentum of the system stays constant to $0$. At time $t$ a big positive charge is placed on each material points. Then you can use this simple model for the bomb: think about $N $ material points which are very close and firm, so the total momentum of the system is $0$. It can be shown that if there are no external forces acting on a system of $N$ material points then the total momentum of the system stays constant (I assume you know the difference between internal and external forces). Since, per Newton’s second law, the force on an object equals its change in momentum, each part undergoes an equal and opposite change in momentum. Per Newton’s third law, part 2 applies an equal and opposite force on part 1 propelling it in the opposite direction. The spring is tripped (explosive ignited) so that part 1 propels part 2 with a force in one direction. Attached to part 1 is a compressed massless ideal spring (mechanical potential energy equivalent of the chemical potential energy of the explosive) and part 2 is in contact with the other end of the spring. You can also think of the original part prior to the explosion as consisting of two parts. An example is the explosion propelling a bullet giving it momentum in one direction and the recoil force of the bullet on the gun giving the gun equal momentum in the opposite direction. The source of the forces are the rapid release of heat and large quantities of high pressure gases. From this we can say that, in the case of an exploding part, each fragment of the part exerts an equal and opposite force on the other fragment causing them to separate with equal and opposite momenta. ![]() Simply stated, Newton’s third law says for every force there is an equal and opposite force. ![]()
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