@classmethod
def check_collision(cls, ball1, ball2):
    """Return true if colliding with another ball"""
    norm = (ball1.positions[-1] - ball2.positions[-1]) ** 2
    norm = norm[0] + norm[1]
    return not (ball1.static and ball2.static) and norm < (ball1.radius +
        ball2.radius) ** 2
