I am a newbie in coding and using Processing. I am doing my project for my University Course based on Bouncing bubbles tutorial on Processing website ( http://processing.org/learning/topics/bouncybubbles.html).

I added more codes to make the balls disappear when I scroll mouse wheel down and I make them appear again when I scroll mouse wheel up. However, after making the balls appear, some of them just pass through each other and some of them bounce.

In addition, I still don't know how to control the speed of the balls at the beginning. Sometime they move really fast, and sometime they are just frozen. 

This is my project file:  http://www.mediafire.com/?o1ek5356xijloft

I appreciate if anyone can show me how to fix those problems.

Thanks for reading my post.