题目

阅读下面短文，在空白处填入1个适当的单词或括号内单词的正确形式。 How is your everything going? I am writing toturn to you for help in that I'ma messhere. Asmember of our school basketball team, I amcrazy about basketball and pretty good at it, isprobably why I was so mad when we lost our last match. At first, we played well,but I felt that the team (let)down by our point guide, a key player., it was like he wasn't even on the court!Disappointed by his (behave),I said all this to my best friend. I was just letting off steam really, becauseI was so angry, but then （15分）某校物理兴趣小组举行遥控车比赛，比赛路径如图所示．赛车从A点出发，沿水平桌面运动的距离后从B点飞出，过B点时赛车电源自动关闭，越过壕沟后进入光滑的轨道CD，由D点进入半径为R的光滑竖直圆轨道，离开圆轨道后，继续在光滑平直轨道上运动．图中L=4m，h=1.25m，s=1.5m，R=0.32m．已知赛车质量m=0.1kg，赛车通电后受到恒定的牵引力，在水平桌面上受到阻力恒为0.3N，其它过程阻力不计，赛车越过壕沟进入光滑轨道CD时不反弹且无能量损失，（g取10m/s2）．求：（1）赛车要安全越过壕沟，在B点的速度至少多大？（2）赛车要安全通过竖直圆轨道，在D点速度至少多大？（3）赛车要完成比赛，赛车的牵引力至少多大？