remixing it for gravity field and potential  work in progress :)  

1 convert all variables to mass
2 convert constant to G  
3 redraw the position of the force
4 introduce range and rangey to make the mass always appears as a circle as a function of the xmax, xmin, ymin, ymax used by applet
5 trying to convert the symbolic representation to mass type from electric type
6 added ODE to make test mass move according to d(xtest)/dt = vxtest & d(vxtest)/dt = g_net
7 added code to make vxtest=0 when slider move or xtest drag etc.
8 change all text to gravity instead of electric.
9 added Fnet = mtest*g 10 added U = mtest*ø

10 play button for ODE to run
11 added slider from text mass
12 changes are made based on an applet by Andrew Duffy is the creator of his original applet.
13 closer remix was based on
12 May 2011
added 0.00 E-08 to all readings to make it as calculated by calculator
July 2011
made the slider to include input fields for exact value of mass = 300 kg for example
some slight adjustment of the check-box
03 august 2011
added step () button
move checkbox to left of bar value for both potential and field to reduce space taken up by the words for the checkboxes

To do.
May need one customized to real earth and moon data
Mass of Earth = 6.0 x 1024 kg
Radius of Earth = 6.40 x 10^6 m

Mass of Moon = 7.35 x 10^22 kg
Radius of Moon’s orbit = 3.84 x 10^8 m

Time for Moon to complete one orbit around the Earth = 2.36 x 106 s