1. The problem statement, all variables and given/known data A neutron collides elastically with a helium nucleus (at rest initially) whose mass is four times that of the neutron. The helium nucleus is observed to rebound at an angle Theta2 = 45° from the neutron's initial direction. The neutron's initial speed is 6.6x 10^5 m/s. Determine the angle at which the neutron rebounds, O, measured from its initial direction. What is the speed of the neutron after the collision? What is the speed of the helium nucleus after the collision? 2. Relevant equations Conservation of KE Conservation of momentum 1/2M1V1^2= 1/2M1V'1^2 + 1/2M2V'2^2 M1V1=M1V'1*cosO + M2V2cos45 0=M1V'1*sinO + M2V'2sin45 M2=4M1 V1=V'1 + 2*V'2 V1=V'1*cosO + 4*V'2*cos45 V'1*sinO= 4*V'2*sin 45 3. The attempt at a solution I have 3 equations and 3 unknows (V'1, V'2, O) I tried solving for O and it becomes a huge mess. Is there a simple way to do this? Or do I really have to figure out all this mess? Any help and tips are appreciated.