For example, consider a bullet (moving horizontally) that impacts a block (at rest) on an incline. From what I know, momentum1 + impulse = momentum2 I have the solution to the problem, and it says (for the impact), (mass of bullet)(speed of bullet) + 0 = (mass of bullet+block)(speed of bullet+block) I don't understand why the impulse is 0 though. According to the video above, which poses a similar problem, he says since we're talking x-direction, no need to worry about the gravity, and normal forces. But in this problem, since it's on an incline, shouldn't the x-component of the gravity be considered? (Note, the positive x-direction is taken to be along the ramp, so gravity considered would be mgsinθ. Is it because the impact time is so minuscule that FΔt = 0?