program fly_drag_anim
      real m,k

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

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

      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

        call pgbbuf
        call pgsci(0)
        call pgcirc(xold,yold,r)
        call pgsci(1)
        call pgcirc(x,y,r)
        call pgebuf

        xold=x
        yold=y
      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