General Physics
posted by Scott .
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 è'2 = 45° from the neutron's initial direction. The neutron's initial speed is 6.6x10^5 m/s. Determine the angle at which the neutron rebounds, è'1, measured from its initial direction.
___°
What is the speed of the neutron after the collision?
___ m/s
What is the speed of the helium nucleus after the collision?
___ m/s

Elastic? Ok, momentum equations and energy equations can be used.
The sum of the final KE =initial KE
The sum of momentums in orig direction=momentum of neutron originally.
The sum of momentum in direction perpendicular to orig direction=zero
You will have three unknowns:
velocity of neutron after
velocity of He after
Theta.
Three equations, three unknowns. A little algebra and trig :) 
thanks but I know that there are three unknowns and three equations. Getting the formulas are the difficult part.