After a firework is launched, (aside from effects of air resistance, and particularly after the thrust from gunpowder is gone) you have simple projectile motion. When it explodes into a shower of pieces, the center of mass of the firework continues to follow the same simple elegant parabolic trajectory it would have, had it been a dud. Newton's Law's (properly applied) continue to hold just the same for the most complex objects (like human bodies ) and even the collection of fragments of a firework that aren't connected to each other in any way! A trick for dealing with more complex systems is to consider two new quantities: center of mass, and momentum. The former is a special point which behaves, in many respects, like "a point mass" even if the object is complex. The latter is a vector property of any system, a quantity which will allow us to extend Newton's Second Law in even more general and powerful ways.