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2001年高考上海卷第13题是一道联系社会生活实际的好题,它涉及到用现代科技进行交通监测。该题本应只有一个答案,而评分标准中提供了两个答案(17m/s,17.9m/s),这究竟是怎么回事呢? 图1是在高速公路上用超声波测量仪测量车速的示意图。测速仪发出并接收超声波脉冲信号,根据发出和接收到的信号间的时间差,测出被测物体的速度。图2中p_1和p_2是测速仪发出的超声波信号,n_1和n_2分别是p_1和p_2由汽车反射回来的信
The thirteenth question of the 2001 Shanghai Exam on College Entrance Examination is a good topic that relates to the reality of social life. It involves the use of modern science and technology for traffic monitoring. There should be only one answer to this question, and the scoring standard provides two answers (17m/s, 17.9m/s). What’s going on here? Figure 1 shows the speed measurement on an expressway using an ultrasonic measuring device. Schematic diagram. The speedometer sends and receives ultrasonic pulse signals, and measures the speed of the measured object according to the time difference between the transmitted and received signals. In Figure 2, p_1 and p_2 are the ultrasonic signals emitted by the speedometer. n_1 and n_2 are the letters of p_1 and p_2 reflected by the car, respectively.