program fly_drag_anim_arrow
      real m,k
      real xnew(13),ynew(13),xold(13),yold(13)
      real xa(13),ya(13)

 1000 write(*,*)'k=?'
      read(*,*)k
      write(*,*)'m=?'
      read(*,*)m

      write(*,*)'vx?'
      read(*,*)vx
      write(*,*)'vy?'
      read(*,*)vy

      xa(1)=2
      ya(1)=0
      xa(2)=1
      ya(2)=1
      xa(3)=1
      ya(3)=0
      xa(4)=-1
      ya(4)=0
      xa(5)=-1.5
      ya(5)=1
      xa(6)=-2
      ya(6)=1
      xa(7)=-1.5
      ya(7)=0
      xa(8)=-2
      ya(8)=-1
      xa(9)=-1.5
      ya(9)=-1
      xa(10)=-1
      ya(10)=0
      xa(11)=1
      ya(11)=0
      xa(12)=1
      ya(12)=-1
      xa(13)=2
      ya(13)=0

      idummy=pgopen('/xwin')
      call pgenv(0.0,10.0,0.0,10.0,1,0)

      dt=0.00001
      y=1.0
      x=0.0
      ax=0
      ay=-9.8

      r=0.2
      xold=x
      yold=y

      do while (y>0)
        v=sqrt(vx**2+vy**2)
        Fd=(-1)*k*v
        Fdx=Fd*(vx/v)
        Fdy=Fd*(vy/v)
        ax=Fdx/m
        ay=-9.8+Fdy/m
        vx=vx+ax*dt
        vy=vy+ay*dt
        x=x+vx*dt
        y=y+vy*dt

c rotate the object at given angle
        do i=1,13
          xnew(i)=xa(i)*(vx/v)-ya(i)*(vy/v)
          ynew(i)=xa(i)*(vy/v)+ya(i)*(vx/v)
        end do

c position the rotated object
        do i=1,13
          xnew(i)=xnew(i)+x
          ynew(i)=ynew(i)+y
        end do

        call pgbbuf
        call pgsci(0)
        call pgline(13,xold,yold)
        call pgsci(1)
        call pgline(13,xnew,ynew)
        call pgebuf

        do i=1,13
          xold(i)=xnew(i)
          yold(i)=ynew(i)
        end do
      end do

      write(*,*) 'distance=',x,'  hight=',y
 2000 write(*,*)'again?(1=Yes,0=No)'
      read(*,*)j
      if(j==1)goto 1000
      if(j.ne.0)goto 2000
c     call pgclos
      end