Impulse
(SFx)*Dt = D(mvx) and (SFy)*Dt = D(mvy)
The quantity on the left side of the above equations also has a name:
impulsex = (SFx)*Dt and impulsey = (SFx)*Dt .
When we have collisions, the forces of collision usually happen in very short times, and the details of these collisions are hard to determine.