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背景外国的教材里有这样一道练习:如图1,两个小钢球分别悬挂在两个电磁铁下,通过开关控制使得两个钢球从不同的高度同
Background There are such exercises in foreign textbooks: As shown in Figure 1, two small steel balls are suspended under two electromagnets, respectively, and the two steel balls are controlled at different heights by switch control.