changes made:

1 key in real data of Earth Moon using new format 0.00E00 like in tracker
2 redrawn and scale the world view to fit both g field graph and potential graph.
3 insert earth and moon images
4 design menu for "Moon Surface;Net Force Zero;Earth Surface;Random"
5 design features for exploring escape velocity concept select position from drop down menu and key in velocity
v_escape_moon = 2280 m s-1
v_escape_earth = 11300 m s-1
6 dt is in 60 sec interval
7 can explore the gravitational constant on Earth surface = 9.81 m/s^2 and Moon =  1.6 m/s^2 approximately