It's Sunday morning. There are many people in the park, some boys are playing basketball. There are some girls under a big tree. They're singing and dancing. What are those women doing? They are drinking tea. Look at the woman in a blue coat. Who's she? She's my mother. She is talking to Edward. Edward is her student. He is a good student. He studies hard. He's good at all the subjects. And he's friendly to all his teachers and his friends.

如图所示，一小球m，用长L的悬线固定与一点O，在O点正下方 $\frac{L}{2}$ 处有一长钉．把悬线沿水平方向拉直后无初速度释放，当悬线碰到钉子瞬间（   ）