Despite our inability to solve the archer problem using our techniques learned so far, this is a very simple problem to solve if we introduce a new quantity that describes motion, linear momentum. Let us apply the General Problem-Solving Strategy and conceptualize an isolated system of two particles (Fig.9.1) with masses m1 and m2 and moving with velocities V1 and V2 at an instant of time. Because the system is isolated, the only force on particle is that other and we can categorize this as a situation in which Newton’s laws will be useful. If a force from particle 1 (for example, a gravitational opposite in direction-that particle 2 exerts on particle 1. That is, they from a Newton’s third law action-reaction pair, so that F12 = -F21. We can express this condition as