Question

A 12000 kg train engine moving at 2.2 m/s locks into 3 boxcars with a total mass of 25000 kg sitting still. If the collision is inelastic and has little friction, how fast will the engine and cars move?

I got 1.4 but correct answer is 0.71?
I don't understand how
12000x2.2 +25000x0 =26400

12000+25000=37000/26400
=1.4

Answer 1

momentum is conserved

(12000 * 2.2) + (25000 * 0) = (12000 + 25000) * v

Answer 2

so I should set this up at initialV=combined and divided by the combined mass

That's where I'm messing it up.

thanks