Tracker 4.85 features demo by douglas brown

Tracker 4.85 new features: http://youtu.be/MAD2Jd_6s7Q
New features include 

1. a translucent grid overlay, mouse control of numerical values in the model builder, 
2. model "launching" with a point mass, and 
3. easy font resizing for presentations or hi-res monitors

thanks professor douglas brown for creating this wonderful tool.