function zp = hidrodinamico(t,z)
  m = 1;
  b = 1;
  g = 10;
  V = sqrt(z(2)^2+z(4)^2);
  zp(1) = z(2);
  zp(2) = -b/m*V*z(2);
  zp(3) = z(4);
  zp(4) = g - b/m*V*z(4);
endfunction