A 5 kg object and a 12 kg object are connected by a massless compressed spring and rest on a frictionless?

Question

After the spring is released, the smaller mass has a velocity of 9 m/s to the left. What is the velocity of the object with the larger mass?

Answer 1

m1v1 + m2v2 = 0
5kg * 9m/s = - 12kg x V
V = -3.75m/s

or the larger object has a velocity of 3.75m/s to the right.

Answer 2

Conservation of momentum.

initial momentum = 0
final momentum must be 0

m1*v1 + m2*v2 = 0; you have m1, m2 and v1, solve for v2