Are they catching butterflies? [ ]
A. No, they aren't.
B. Yes, they do.
C. Yes, he is.
答案:A
系统压力下降有哪些原因?
某同学在做单臂俯卧撑运动,如图所示.他的重心在A点,所受重力为520N,他将身体撑起处于平衡状态时,地面对手的支持力为______N.若他1min内完成30个俯卧撑,每次重心上升的高度是0.2m,则他做俯卧撑的功率是______W.