

Initial momentum of the system (parent nucleus) = 0
Let v1 and v2 be the respective velocities of the daughter nuclei having masses m1 and m2.
Total linear momentum of the system after disintegration = m1v1 + m2v2
According to the law of conservation of momentum:
Total initial momentum = Total final momentum
0 = m1v1 + m2v2
v1 = -m2v2 / m1
Here, the negative sign indicates that the fragments of the parent nucleus move in directions opposite to each other.
