How can I accurately model gravity using vector fields in Matlab?

[Dirty D]

I am trying to model gravity with vector field lines in Matlab. I can kinda do it with a 2D plot, but I can't quite grasp the "scale" field in the quiver function. I then tried to model in 3D vectors and I just made a mess w/ that. Below is my simple 3D and 2D code.

Help w/ this code or if you know a better way to do this would be nice!

non working 3D plot
[X Y Z] = meshgrid(-3:.1:3);
r = sqrt(X.^2 + Y.^2 + Z.^2);
ag = 1./r.^2;
[ax ay az] = gradient(ag);
quiver3(X, Y, Z, ax, ay, az)

Poor 2D plot
M = 1; %Mass of Earth in kg
G = 1; %Gravitational Constant
R = 1; %Radius of Earth in m
[x y] = meshgrid(-3:.75:3);
r = sqrt(x.^2 + y.^2);
ag = G*M./r.^2;
[ax ay] = gradient(ag);
contour(x,y,r,[R R])
colormap cool
hold on
scale = 0;
quiver(x,y,ax,ay,scale)
hold off

 

[Integral]

Prehaps you could post an image file of some sort. I cannot open your .m file and will not aprove it without doing so.

 

[Dirty D]

is this better?