Pygame:我怎样才能纠正掉下来的球向上移动？

import pygame
from pygame.locals import *

pygame.init()

screen_width, screen_height = 800, 600
screen = pygame.display.set_mode((screen_width, screen_height))
clock = pygame.time.Clock()

ball_pos = pygame.math.Vector2(screen_width/2, screen_height/2)
ball_speed = pygame.math.Vector2(0, -10)

while True:
    for event in pygame.event.get():
        if event.type == QUIT:
            pygame.quit()
            sys.exit()

    # 更新球的位置
    ball_pos += ball_speed

    # 检测球是否触碰到屏幕顶部边界
    if ball_pos.y <= 0:
        ball_speed.y *= -1  # 反向速度向量使球向上移动

    screen.fill((255, 255, 255))  # 清空屏幕

    # 在屏幕上绘制球
    pygame.draw.circle(screen, (255, 0, 0), (int(ball_pos.x), int(ball_pos.y)), 10)

    pygame.display.flip()
    clock.tick(60)