11. A neutron collides elastically with a Helium nucleus that is at rest. The Helium nucleus has four times the mass of the neutron and is observed to fly off at an angle of 45°
from the direction of the initial neutron
. Determine the angle of the neutron’s final velocity with respect to its initial path, and the final
speeds of the neutron and the Helium nucleus. The neutron’s initial speed is
.

Question

11. A neutron collides elastically with a Helium nucleus that is at rest. The Helium nucleus has four times the mass of the neutron and is observed to fly off at an angle of 45° 
from the direction of the initial neutron
. Determine the angle of the neutron’s final velocity with respect to its initial path, and the final 
speeds of the neutron and the Helium nucleus. The neutron’s initial speed is 
.

jamesj · Answer

Isn't this just a question of using conservation of momentum?