A 30 ton rail car and a 100 ton rail car initially at rest a

A 30 ton rail car and a 100 ton rail car, initially at rest, are connected together with a giant but massless compressed spring between them. When released, the 30 ton car is pushed away at a speed of 4.2 m/s relative to the 100 ton car.

What is the speed of the 30 ton car relative to the ground?

Solution

initial momentum = 0

30v1 + 100v2 = 0

v1 = v2 + 4.2

30v1 + 100(v1 -4.2) = 0

130v1 - 420 = 0

v1 = 3.23 m/s