汽车车厢顶部悬挂一根轻质弹簧，弹簧下端栓一个质量为m的小球。当汽车以某一速率在水平面上匀速行驶时弹簧的长度为L1，当汽车以同一速率匀速通过一个桥面为圆弧形的凸桥最高点时，弹簧长度为L2，则下列选项中正确的是
A．L1= L2
B．L1> L2
C．L1<L2
D．前三种情况都有可能
答案：B
题目分析：水平面上匀速行驶时不需要提供向心力，所以此刻弹力等于重力。但是汽车以同一速率匀速通过一个桥面为圆弧形的凸桥最高点，由于需要指向圆心的向心力，所以重力大于向心力，所以L1> L2，答案为B
点评：本题考查了向心力来源的求法。在分析圆周运动知识时，要对物体受力分析，找出指向圆心的合外力。
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| A young man was going to 1 his holiday in a mountain village. That night he stayed at a small hotel near the train station. 2 going to bed, he went to the owner of the hotel and said, "Excuse me, sir. Will you please 3 me up at a quarter to five tomorrow morning? I'll take the five's 4 " " Oh, sorry," the owner said 5 a hurry, "I am afraid I can't. I won't be able to get up so 6 ." The young man was about to return to his 7 when he stopped and asked, "Do you have an alarm-clock? 8 it can help me." "Yes, here you are, young man." The young man thanked the owner happily. But when he looked at the alarm-clock closely, it seemed there was something 9 with it. "Will it ring 10 time?" The young man asked. "Sure! You just give it a good shake at a quarter to five. And it will ring." | ||||
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