What Is the Final Velocity After Two Freight Cars Collide and Interlock?

[zelda1850]

Homework Statement

A 6500 kg freight car traveling at 2.5 m/s collides with an 8000 kg stationary freight car if they interlock upon collision find their velocity

Homework Equations

M1 = 6500
M2 = 8000 kg
V1 = 2.5 m/s
V2= ?

i think the equation is p before = p after but I am confused how i can use it

The Attempt at a Solution

not sure how to use the equation

 

[rock.freak667]

Linear Momentum is mass x velocity. So before collision, what is the total momentum? After collision, the cars lock together, so what is the new mass and hence the final momentum?