
When a ball collides with a wall, at an angle θ say, the impulse exerted on the ball is perpendicular to the wall and causes a change in the momentum of the ball in that direction; it does not however affect the momentum parallel to the wall.
Therefore, if the approach velocity of the sphere is resolved into components parallel and perpendicular to the wall, one of these components is changed by the impact and the other remains unchanged.
Input speed of blue ball.  
If two similar balls are free to move on a horizontal surface and collide when their velocities are not in the same straight line, the two impulses that act on impact are perpendicular to the common tangent of the two balls. They lie on the line joining the centres of the balls.
Where j is the impulse with u and v being the velocities of the balls.
Therefore, for each ball there is a change in momentum (and speed) along this line of centres, but not perpendicular to it.
Experiment with this applet that collides 2 balls. You can change the horizontal speeds of the balls by entering your own values in the two boxes provided in the applet. (a negative value represents a speed in the left direction)
Oblique Impact Example Applet   
