Physical systems mimic the way objects around us react to each other - just think of bouncing balls and you've got it.
In computer games, physics is everywhere, making games more complex and realistic all the time, with Box2D being one of the more well-known physics libraries. Fortunately for us it's been ported to AS3, but while it takes a lot of the hard work out of 2D physics, it's still pretty complex.
Zevan Rosser's QuickBox2D sits on top of Box2D and makes it easier still, so we can just get stuck in without having to worry too much about the complexities of how the system works. We'll set up a simple system with QuickBox2D and paint its trails to a canvas, grabbing colours from a photo as we go and injecting energy into the system with the mouse.