1. Two railroad cars traveling at 5 m/s collide with three railroad cars at rest. The cars link together and move further down the track. Assume each railroad car is identical to each other. What is the speed after the collision of all five cars. This is the question I have and I see I need to use the equation: Vf = m1V1i+m2V2i/m1+m2, I have the initial velocity 1 as 5 m/s and the initial velocity 2 as 0 but how do i express the masses.......I appreciate your help.