Question

A submarine of mass 2.55 × 106 kg and initially at rest fires a torpedo of mass 254 kg. The torpedo has an initial speed of 100 m/s. What is the initial recoil speed of the submarine? Neglect the drag force of the water.
Damon

conserving momentum (Initial is zero):
2.55*10^6 v + 254 (100) = 0
v is negative because if it fires out the bow, the sub moves backwards.