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QUESTION: Pls sir, how can we know the body that wil hav the negative velocity before collision, when calculating the momenta(um) of two coliding mases in oposite directions to each other before collision. For example; if 2 mases 2kg & 6kg travel wit 12m/s & 4m/s.

ANSWER: If you have a 2kg mass traveling east at 12m/s colliding with a 6kg mass traveling west at
4m/s, then they will both stop and the velocity will be zero

---------- FOLLOW-UP ----------

QUESTION: Thanks sir, but what if the values were different, let's say; 2kg mass has speed of 10m/s and the 6kg has speed of 15m/s

Answer
Momentum is conserved in a collision.  It just happened that your example had equal momenta in opposite directions so that the sum was zero.  
For your second example, you have 20kgm/s to the right and 90kgm/s to the left for a net
momemtum of 70kgm/s to the left.  That means the 2kg mass is going to reverse direction and be going (7/4)m/s to the left and the 6kg mass is going (7/2)m/s to the left  

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Richard J. Raridon

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I can answer most questions in undergraduate physics courses, including electricity and magnetism, atomic and nuclear, mechanics and optics.

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I have taught undergraduate physics courses

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Sigma Xi, AAAS, SE section of APS

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BA in math, MA in physics, PhD in physical chemistry

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