Question

A railway wagon of mass 15 t is moving along a level track at 10 km/h when it collides and couples together with a second wagon of mass 30 t moving in the same direction at 5 km/h. if the two wagons couple together after the impact, what would be their common velocity?
just conserve momentum before and after the collision. You then have

15*10 + 30*5 = (15+30)v
But don't you convert it into the base units?

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