A couple of months ago I ran across a free download for a software called Phun. Phun is described as a 2-D physics sandbox. You can see what sort of things can be done using the program on the video below.
I'm always up for trying out free software and we've found some great ones out there. This one is a keeper and enjoyed by both of my kids. I love the creativity it inspires! You can download your own copy at http://www.phunland.com/wiki/Home. Enjoy!