A He4 nucleus, with a mass of 4 amu moving with speed v breaks up into a neutron 1amu and a He3 nucleus 3amu . If the neutron moves in a direction perpendicular to the direction of motion of the original He4 nucleus with speed 3v, what is the speed of the He3 nucleus? if the neutron moves down. The nucleus will be pushed upward with 1/3v. the nuetron also has the original 1v in the horizontal direction. so should i just use pythagorean theorem?: so i get sqrt(10)v/3?