an empty 2,500 kg train car is headed northbound at a velocity of 5 m/s. ahead of the first car, an empty 1,500 kg car is headed northbound on the same track at 1 m/s. the faster train bumps into the slower one, and they combine into one train. what is the velocity of the two-car train? 1 m/s 3 m/s 3.5 m/s 5 m/s

Here since during collision of two trains there is no external force on it.

Since there is no force on it so momentum of two trains before collision and after collision will remain conserved

we can write the equation of momentum conservation as

given that

now by the momentum conservation

so the two trains will combine and move with speed 3.5 m/s