Question

“引体向上”是同学们常做的一项健身运动。如图甲所示，质量为m＝50kg的某同学两手正握单杠，手臂完全伸直，身体呈自然悬垂状态，此时他的下颚距单杠的高度为H＝0.4 m，然后他用F＝600 N的恒力将身体向上拉至某位置时不再用力，身体依靠惯性继续向上运动，为保证其下颚超过单杠达到合格要求，如图乙所示。恒力F的作用时间至少为多少？不计空气阻力，取重力加速度g＝10 m/s²。

Answer 1

t="0.58" s

试题分析：恒力F作用过程
                                                       ①                                                            2分
                                                               ②                                                            2分
                                                                      ③                                                            1分
撤去F后
                                                              ④                                                            1分
又                                                       ⑤                                                            2分
解得t="0.58" s（或s）                          ⑥                                                             2分
点评：关键是找出第二次引体向上成功的临界条件即末速度等于零．是一道好题．