A ball moving with a velocity of 9 meters per second collides with another similar stationary ball. After the collision both the balls move in direction making an angle of 30 degree with the initial direction. After the collision what will the speed of the two balls be?

I used conservation of linear momentum:

$$m9i=2m (\frac {\sqrt3 }{2 }vi+\frac {1}{2}v)j $$

Where $v$ is the final speed and $i$ and $j$ represent unit vectors.

But the answer doesn't come out correctly. What am I doing wrong?

edited Nov 25, 2016 by sammy gerbil