A neutron moving with a speed v collides head on  with a stationary α-particle. The speed of the neutron after collision is

Given mass of neutron = m m Mass of o-particle = 4 m Here, we have the velocity of neutron after collisionv1=m1−m2m1+m2v=m−4mm+4mu=−35v Negative sign indicates that the neutron will move in  opposite direction