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quirck
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Homework Statement
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?
Homework Equations
Impulse mass * Velocity
Kinetic energy mass * velocity^2
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.