1. The problem statement, all variables and given/known data 1 mass M1 of 1 kg moves frictionless at V1 1 meter per second to the right. It fuses with mass M2 of 2 kg and together they keep moving frictionless to the right. What is the speed V2 of the fused objects? 2. Relevant equations Impulse mass * Velocity Kinetic energy mass * velocity^2 3. The attempt at a solution Conservation of energy 1/2*M1*V1*V1 = 1/2 [Joule] = 1/2*(M1+M2)*V2*V2 V2=SQRT (1/3) [m/s] Conservation of momentum M1*V1 = (M1+M2)*V2 V2=1/3 [m/s] My confusion is. When and why should I conserve momentum and when and why should I conserve energy? I think I can imagine what is energy of a moving object, however I think that somehow I cannot imagine what is momentum.