I have read that when a bomb explodes into fragments, then total momentum of the system remains conserved. And law of conservation of momentum is based on newton's second and third law. So I am not able to understand or get an intuition of how Newton's third law applies on explosion.
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Simply stated, Newton’s third law says for every force there is an equal and opposite force. 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. The source of the forces are the rapid release of heat and large quantities of high pressure gases. 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.
You can also think of the original part prior to the explosion as consisting of two parts. 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.
The spring is tripped (explosive ignited) so that part 1 propels part 2 with a force in one direction. Per Newton’s third law, part 2 applies an equal and opposite force on part 1 propelling it in the opposite direction. 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.
Hope this helps.
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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).
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 $0$. At time $t$ a big positive charge is placed on each material points. There will be some strong repulsion forces, but all of these are internal forces, so the total momentum of the system stays constant to $0$.
Edit:
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. 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. This is true only in the first instants after the explosion, since after there are external forces given by friction which act both on the shockwave and on the fragments.
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Since you want an intuitive understanding, I suggest a "thought experiment".
Suppose I put a huge plastic sphere, 200m diameter, deep into space. No planets or anything else near it. Zero velocity relative to your ship, as measured by your technical expert.
Right in the middle of the sphere, I put a bomb on a timer. The bomb is pretty big, but not big enough to break the sphere if it blows up right at the middle of it.
The timer comes to zero, and the bomb goes boom!
Which way does the sphere move?
Well, if you think about it, it might shake a bit from it, but it's not going to suddenly start moving in any direction.
The sphere stays in the same place. It started with zero velocity relative to the ship. It ended up with the same. Started same mass, and ended same mass. So no momentum change.
If you saw the explosion in ultra slow motion, at a microscopic level, you'd see atoms and molecules rearranging themselves incredibly fast, and a lot of heat and gas resulting. But it's like when you blow air in in a balloon. The air all presses against each other. Equal and opposite. Each tiny bit that flies away as part of the explosion, also "kicks off" away from other tiny parts that start to move in the other direction.
This happens billions upon billions of times in a tiny fraction of a second, making an explosion.
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I am not sure, given the generic question, what exactly you do not understand of this. The other answers have made points how this can be seen at a technical level, stressing the fact that there can be internal forces at play. I'd like to give two more input:
- Force is a change of momentum over time, therefore if you look at the sphere as made of two halves, and you separate the two half-spheres by any force you need to change their momentum.
- The explosive is an external source of energy. Indeed the kinetic energy of the whole ball at rest before the explosions is zero, whereas the kinetic energy of the two pieces after the explosion is greater than zero.
These two points are somewhat not clear in textbooks or anyways easy to be missed at first. I hope that helps.
