I am not looking for homework help on how to do the math for vectors and momentum. Nor do I expect to understand the answer theoretically, but is/has any work been done that examines what it is that "carries" or encodes or transfers momentum? By this I mean that a particle receives energy from another particle and it moves in a particular direction. How does that particle "know" to continue in a particular direction. I know it's momentum and that the particle has to move, but why always in the same direction? Of course the flip side of that question is "why not?" for which I have no answer.