Many questions involving forces can’t be answered by directly applying Newton’s second law, ΣF = ma. For example, when a truck collides head-on with a compact car, what determines which way the wreckage moves after the collision? In playing pool, how do you decide how to aim the cue ball in order to knock the eight ball into the pocket? And when a meteorite collides with the earth, how much of the meteorite’s kinetic energy is released in the impact? All of these questions involve forces about which we know very little: the forces between the car and the truck, between the two pool balls, or between the meteorite and the earth. Remarkably, we will find in this unit that we don’t have to know anything about these forces to answer questions of this kind! Our approach uses two new concepts, momentum and impulse, and a new conservation law, conservation of momentum.