program fall_polt_drag
      real k1,m

      write(*,*)'initial position ?'
      read(*,*)x
      xini=x
      write(*,*)'initial hight ?'
      read(*,*)y
      write(*,*)'Vx ?'
      read(*,*)vx
      write(*,*)'Vy ?'
      read(*,*)vy
      write(*,*)'air drag coefficient?'
      read(*,*)k1
      write(*,*)'Mass of the object?'
      read(*,*)m

      if(pgopen('/xwin')<=0)stop
      call pgenv(0.0,10.0,0.0,10.0,1,0)

      dt=0.0001
      do while(y>0)
        v=sqrt(vx**2+vy**2)
        f_d=(-1)*k1*v
        f_dx=f_d*vx/v
        f_dy=f_d*vy/v
        ax=f_dx/m
        ay=-9.8+f_dy/m
        vx=vx+ax*dt
        vy=vy+ay*dt
        x=x+vx*dt
        y=y+vy*dt
        call pgpt(1,x,y,20)
      end do

      write(*,*)'Final hight =',y,'meters.'
      write(*,*)'Final distace =',x-xini,'meters.'

      call pgclos

      end