The Rutherford experiment

1. Apr 13, 2008

physco827

The Rutherford experiment
An alpha particle (a helium nucleus, containing 2 protons and 2 neutrons) starts out with kinetic energy of 10.2 MeV (10.2 106 eV), and heads in the +x direction straight toward a gold nucleus (containing 79 protons and 118 neutrons). The particles are initially far apart. Answer the following questions about the collision.
What is the initial momentum of the alpha particle? (You may assume its speed is small compared to the speed of light).

What I found:
alpha,i = < 1.4733e-19 , 0, 0 > kg·m/s
What is the initial momentum of the gold nucleus?
Au,i = < 0 , 0 , 0 > kg ·m/s

What i can't get:
What is the final momentum of the alpha particle, long after it interacts with the gold nucleus?
alpha,f = < ? , 0 , 0 > kg·m/s
What is the final momentum of the gold nucleus, long after it interacts with the alpha particle?
Au,f = < ? , 0 , 0 > kg·m/s

I tried conservation of momentum but all the momentums cancel themselves out. I don't understand how to get the final momentum of the alpha particle.