2d-astrodynamics 2D astrodynamics. http://lucas-tulio.github.io/2d-astrodynamics/ Sources http://www.physicsclassroom.com/class/circles/Lesson-3/The-Value-of-g http://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation http://www.grc.nasa.gov/WWW/k-12/airplane/wteq.html