interactive textbook prototype chapter on gravity advanced level physics. contains many simulations or models on gravity topics such as 2 body orbits, field strength, potential escape velocity, geostationary orbits (may require modern browser), Earth Moon System, Kepler System only available on Java. many thanks to Open Source Physics/Easy JavaScript Simulations. based on lectures notes from Yishun Junior College, Singapore. photo from Leong Tze Kwang. Licensed Creative Commons Attribution ShareALike CC-BY-SA