program dragged_fall
      real xs(4),ys(4),xsold(4),ysold(4),xp(4),yp(4)
      real xpold(4),ypold(4)
      xs(1)=0.0
      ys(1)=0.1
      xs(2)=0.4
      ys(2)=0.0
      xs(3)=0.0
      ys(3)=-0.1
      xs(4)=xs(1)
      ys(4)=ys(1)
      do i=1,4
        xsold(i)=xs(i)
        ysold(i)=ys(i)
      enddo
      x=0.0
      y=2.0
      xold=x
      yold=y
      vx=3.0
      vy=0.0
      ax=0.0
      ay=-9.8
      k = 1.0
      m = 2.0
      if(pgopen('/xwin')<=0)stop
      call pgenv(0.0,2.0,0.0,2.0,1,0)
      dt=0.00001
      do i=1, 1000000
        v = sqrt(vx**2+vy**2)
        fd = (-1)*k*v
        ax = (fd/m)*(vx/v)
        ay = (fd/m)*(vy/v) -9.8
        vx = vx + ax*dt
        vy = vy + ay*dt
        x = x + vx*dt
        y = y + vy*dt
        do j=1,4
          xp(j)=xs(j)+x
          yp(j)=ys(j)+y
          xpold(j)=xsold(j)+xold
          ypold(j)=ysold(j)+yold
        enddo
        call pgbbuf
        call pgsci(0)
        call pgline(4,xpold,ypold)
        call pgsci(1)
        call pgline(4,xp,yp)
        call pgebuf
        xold=x
        yold=y
      enddo

      call pgclos
      end