program arrow_anim
      real x(7),y(7),x_rot(7),y_rot(7),xold(7),yold(7)
      real k,m

      x(1)= 1.0
      y(1)= 0.0
      x(2)= 1.0
      y(2)= 0.3
      x(3)= 2.0
      y(3)= 0.0
      x(4)=-2.0
      y(4)= 0.0
      x(5)=-2.5
      y(5)= 0.5
      x(6)=-1.5
      y(6)= 0.5
      x(7)=-1.0
      y(7)= 0.0

      write(*,*)'Vx=?'
      read(*,*)vx
      write(*,*)'Vy=?'
      read(*,*)vy
      write(*,*)'k=?'
      read(*,*)k
      write(*,*)'m=?'
      read(*,*)m
      xcm=0.0
      ycm=1.0
      dt=0.00001
      i_dummy=pgopen('/xwin')
      call pgenv(0.0,10.0,0.0,10.0,1,0)
      do while (ycm>=0)
        v=sqrt(vx**2+vy**2)
        fd=(-1)*k*v
        ax=fd*(vx/v)/m
        ay=-9.8+fd*(vy/v)/m
        vx=vx+ax*dt
        vy=vy+ay*dt
        xcm=xcm+vx*dt
        ycm=ycm+vy*dt
        do i=1,7
          x_rot(i)=x(i)+xcm
          y_rot(i)=y(i)+ycm
        end do
        call pgbbuf
          call pgsci(0)
          call pgline(7,xold,yold)
          call pgsci(1)
          call pgline(7,x_rot,y_rot)
        call pgebuf
        xcmold=xcm
        ycmold=ycm
        do i=1,7
          xold(i)=x_rot(i)
          yold(i)=y_rot(i)
        end do
      end do
      write(*,*)'Distance=',xcm,';Hight=',ycm
      call pgclos
      end