引力の例
Private Sub gravitation(x0, y0, g)
　 Dim L2 As Double
　 Dim dx, dy As Long
　 dy = ball_y - y0
　 dx = ball_x - x0
　 L2 = g / (dx * dx + dy * dy)
　 vx = vx - dx * L2
　 vy = vy - dy * L2
End Sub

呼び出しは BallMove 手続き内で
gravitation 100, 70, 0.3
みたいに