题目

如图所示，一质量为 m的汽车保持恒定的速率运动，若通过凸形路面最高处时对路面的压力为 F 1 ，通过凹形路面最低处时对路面的压力为 F2 ，则（ ）A． F 1 = mg B． F 1 >mg C． F 2 = mg D． F 2 >mg 答案：D【详解】 汽车通过凸形桥和凹形桥时均做圆周运动，因此要对汽车进行正确的受力分析，然后利用向心力公式求解． 设汽车通过凹形桥最低点和凸形桥最高点时所受支持力分别为 和 ，根据向心力公式有： 通过凹形桥最低点时 ， 通过凸形桥最高点时： ， 由于 m、 v 、 r 均相等，固有 ，根据牛顿You’re rushing to work and a man ahead of you collapses on the sidewalk. Do you stop to help? In a study of by-standers, it was found that some people avert their gaze and keep on  walking rather than stop and get involved. “There is a tendency to decide that no action is needed.” says a psychologist. “The first thoughts that pop into your mind often keep you from offering help. In order to take action, you have to work against them.” Here are some common thoughts that might prevent you from helping. ● Why should I be the one? I’m probably not the most competent person in this crowd. You might think someone older or with more medical knowledge should offer assistance. ● What if he doesn’t really need my help? The fear of embarrassment is powerful; no one wants to risk looking foolish in front of others. ● No one else looks concerned- this must not be a problem. We can follow the people around us, but most people tend to hold back their emotions in public. “If you spot trouble and find yourself explaining inaction, force yourself to stop and evaluate the situation instead of walking on,” says the psychologist. “Then retry to involve other people; you don’t have to take on the entire responsibility of being helpful. Sometimes it’s just a matter of turning to the person next to you and saying, ‘It looks like we should do something.’ Or asking someone if an ambulance has been called and, if not, to call for one. Once you take action, most people will follow you.” 56. Which is NOT the common thought that stops you from helping others? A.I’m not the very person capable of setting the problenu. B.It looks like we should do something. C.It must not be a problem as no one else is concerned. D.He doesn’t really need my help. 57. According to a study of by-standers, what will some people do when a man ahead falls down on the sidewalk? A.They will call for help and then walk away. B. They will stop and offer help. C. They will turn away their eyes and go on walking. D.They will laugh at him. 58. We learn from the last paragraph that if we spot ttouble,           ． A. we sbould call the ambulance as soon as we can. B. we should take on the whole responsibility and do something alone. C. we should stop and evaluate the situation and try to make other people follow. D.we should turn to other people and ask them to take on the responsibility 59. In order to offer others your timely help, you need to        ． A. ask others for help and call the police B.get along well with the passers-by who spot the trouble C. go directly to the police station D.work against the rirst thoughts that prevent you offering help 60. The main purpose of the text is to tell readers        ． A.to give others a hand                       B. to be more competent C.not to risk looking foolish                D.to stop and evaluate the situation