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九年义务教育三年制初级中学教科书物理第一册(人教版),第33页介绍了一个小实验。做法是:照图1甲那样,把小铃铛拴在线上,线的上端穿
Nine-year Compulsory Education Three-year Junior High School Textbook Physics Book 1 (PEP), page 33 introduces a small experiment. The practice is: as shown in Figure 1A, put the small bell on the line, the upper end of the line