A1,600 kg train car rolling freely on level track at 16 m/s bumps into a 1.0 × 103 kg train car moving at 10.0 m/s in the same direction, and the two latch onto each other and continue together. what is their final speed?

a.) 16 m/s
b.) 6.0 m/s
c.) 10 m/s
d.) 14 m/s

This item is solved through the concept of the conservation of momentum which states that the momentum before and after collision should be equal. 
                                   momentum = mass x velocity
            (1,600 kg)(16 m/s) + (1.0x10^3 kg)(10 m/s) = (1600 + 1000 kg)(x)
The value of x is 13.69 m/s. Thus, their final speed is approximately letter D. 14 m/s. 

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