I’m confused about conservation of momentum. I remember learning that the momentum before a collision is the same as after it: Pinitial = Pfinal . So I do that for a mass M1 colliding with another mass M2 that’s initially at rest, M1 x V1initial + 0 = M1 x V1final + M2 x V2final and I get an equation for the velocity V2final , V2final = (M1/M2) x (V1initial - V1final) . Now let’s say that M2 is so large that in the limit it becomes infinite. In that case, V2final = 0, so that V1final = V1initial . But since M1 bounces off of M2 , shouldn't V1final and V1initial be pointing in opposite directions?