program fly_anim
      real m, k
      write(*,*)'Vx?'
      read(*,*)vx
      write(*,*)'Vy?'
      read(*,*)vy
      write(*,*)'mass?'
      read(*,*)m
      write(*,*)'drag coefficient?'
      read(*,*)k
      x=0.0
      y=1.0
      xold=x
      yold=y
      ax=0.0
      ay=-9.8
      dt=0.00001
      i_dummy=pgopen('/xwin')
      call pgenv(0.0,5.0,0.0,5.0,1,0)
      do while(y>0)
        v=sqrt(vx**2+vy**2)
        fd=(-1)*k*v**2
        ax=(fd/m)*(vx/v)
        ay=-9.8+(fd/m)*(vy/v)
        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,0.05)
          call pgsci(1)
          call pgcirc(x,y,0.05)
        call pgebuf
        xold=x
        yold=y
      end do
      call pgclos
      end