function f = bullet( t,state )
%
% Bullet dynamics is two dimensions
%
xp = state(1);
yp = state(2);
xv = state(3);
yv = state(4);
g = 9.81;
dia = 0.44*2.54e-2;
length = 1.5*2.54e-2;
dens = 8000;
area = pi*(dia/2).^2;
mass = dens*area*length;
cd = 0.3;
h = yp;
f = exp(-h/7400);
rho = 1.2 *f ;
v = sqrt(xv*xv+yv*yv);
ad = 0.5*rho*(area/mass)*v*v*cd;
nx = xv/v;
ny = yv/v;
xa = -nx*ad;
ya = -ny*ad - g;
f = [state(3) state(4) xa ya]';