Elastic collision of neon atom problem

[mithanon]

Homework Statement

A neon atom (m = 20.0 u) makes a perfectly elastic collision with another atom at rest. After the impact, the neon atom travels away at a 57.9° angle from its original direction and the unknown atom travels away at a -45.4° angle. What is the mass (in u) of the unknown atom? [Hint: You can use the law of sines.]

variables
m = neon = 20 u
M = mass of unknown
\theta = 57.9°
\phi = 45.4°
v = initial velocity of neon
v' = final velocity of neon
V = final velocity of unknown

Homework Equations

I've set up conservation of momentum equations for both directions, and the energy equation, but I'm stuck with more unknowns than equations

The Attempt at a Solution

mv²=mv'²+MV²
mv = mv'cos\theta + MVcos\phi
0 = mv'sin\theta - MVsin\phi

using a momentum vector sum diagram and law of sines? I got
mv/sin(180-\theta-\phi)=mv'/sin(\phi)

not sure how that helps but I guess I can then express v' in terms of v, eliminating one variable

no idea where to go from here

 

[hage567]

Everything looks OK so far I think. You can also use the law of sines to get the other "side" of the triangle. The problem is now just one of algebra. I would pick one equation (maybe the energy one), and substitute in for the variables you don't want and then isolate M.