Momentum
For instance, when a gun is fired, a small mass (the bullet) moves at a high speed in one direction.  A larger mass (the gun) moves in the opposite direction at a much slower speed.  The momentum of the bullet and the momentum of the gun are exactly equal in size but opposite in direction.  Using vector addition to add the momentum of the bullet to the momentum of the gun (equal in size but opposite in direction) gives a total system momentum of zero.  The momentum of the gun-bullet system has been conserved.