https://phet.colorado.edu/sims/html/gravity-force-lab-basics/latest/gravity-force-lab-basics_en.html
https://phet.colorado.edu/sims/html/gravity-and-orbits/latest/gravity-and-orbits_en.html
https://docs.google.com/document/d/149jPQJHufzVO001N5xi5O-Wdx3Y82euluO4YEseOS7Y/edit?usp=sharing
questions