(3-1)   ①  Start with Sir Isaac’s second law, which we can state as the familiar equation:

 force = mass x acceleration  (F = ma)

Use this to derive a relationship between the applied force (F) and momentum (p).

②  Now invoke the third law, which states that the forces exerted by the colliding objects upon one another are equal and opposite. Use this, together with your answer to part ①,  to show that if objects A and B collide, d(pA + pB)/dt = 0.

③  If this derivative equals zero, what does that mean about the function pA + pB ?