A 40 ton rail car and a 80 ton rail car initially at rest ar

A 40 ton rail car and a 80 ton rail car, initially at rest, are connected together with a giant but massless compressed spring between them. When released, the 40 ton car is pushed away at a speed of 3.8 m/s relative to the 80 ton car.

What is the speed of the 40 ton car relative to the ground?

Solution

Initial momentum = 0 = 40*v1 + 80*v2 = final momentum

v1 = v2 + 3.8

put the value of v2 in above

40*v1 + 80*(v1 - 3.8) = 0

v1 = 304 / 120 = 2.53 m/s relative to the ground