program fly
      real m, k
      integer pgopen

      write(*,*)'number of gun powder?'
      read(*,*) num_gun_powder
      write(*,*)'shooting angle?'
      read(*,*) angle
      angle = angle/180 * 3.14
      write(*,*)'mass?'
      read(*,*) m
      write(*,*)'air resistance?'
      read(*,*) k


      if(pgopen('/xwin')<=0) stop
      call pgenv(0.0,5.0,0.0,5.0,0,0)


c     Ek = (1/2)m*V*V
      Ek = 1.0 * num_gun_powder
      v_ini = sqrt(2*Ek/m)

      dt = 0.001
      vx = v_ini*cos(angle)
      vy = v_ini*sin(angle)
      x = 0.0
      y = 1.0
      g = -9.8
      do while(y >= 0.0)
        v_current =sqrt(vx**2 + vy**2)
        f_drag = (-1)*k*v_current
        f_drag_x = f_drag * vx/v_current
        f_drag_y = f_drag * vy/v_current
        a_drag_x = f_drag_x / m
        a_drag_y = f_drag_y / m
c       ax = 0
c       ay = g
        ax = a_drag_x
        ay = g + a_drag_y
        vx = vx + ax*dt
        vy = vy + ay*dt
        x = x + vx*dt
        y = y + vy*dt
        call pgpt(1,x,y,20)
      end do
      call pgclos

      write(*,*)'final hight is:',y
      write(*,*)'final distance is:',x



      end