Hello everyone. First post here. I'll go the question straightaway. 1. The problem statement, all variables and given/known data A gun of mass "M" placed on a smooth horizontal surface fires a bullet of mass "m" with a velocity "u" at an angle "θ" with the horizontal (ground). The velocity of the center of mass of the gun+bullet system after firing is (in terms of M,m,u and θ)? 2. Relevant equations I know the velocity of the center of mass is given by the equation v = mv / m 3. The attempt at a solution I expanded the above equation to V = (Mv1 + mv2)/(M+m), where v2 and v1 are the velocities of masses M and m after firing. However, I do not know what values to substitute for v1 and v2. I substituted v2 to be ucosθ as it is along the horizontal, and then used the conservation of linear momentum principle to get v1=-mucosθ/M substituting these values if the original formula, I got a wrong answer. The final answer is supposed to be musinθ/(M+m) What mistake have I done here? Thanks in advance for explaining.