function f = sky_diver( v, t )
%
%  return the downward acceleration (m/sec^2) of a sky diver at time t (sec)
%  with a downward velocity v (m/sec).
%

g = 9.8;   %  acceleration of gravity (m/sec^2)
rho = 1.2; %  mass density of air (km/m^3)
a = 0.5;   %  crosssectional area of sky-diver (m^2)
C_d = 1.0; %  coefficient of drag
M = 100.0; %  mass of diver (kg)

alpha = 0.5*rho*a*C_d/M;

f = g - alpha*abs(v)*v;