1. The problem statement, all variables and given/known data http://hypertextbook.com/physics/mechanics/momentum-energy/problems.shtml It's numerical #1, filling out the table, the part I am stuck on is recoil velocity. I have figured out the bullet momentum and bullet energy, however. 2. Relevant equations (m1+m2)v=m1v1'+m2v2' 3. The attempt at a solution The part that I don't understand is what is m2? M1 would be the mass of the bullet, but is the mass of the gun suppose to be m2?