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Practice 4  (1) Figures indicate an ever-increasing crime rate, but it is only too easy to imagine “It will never happen to me.” Unfortunately, statistics show that it really can happen to you and, if you live in a large city, you run twice the risk of be

Practice 4  (1) Figures indicate an ever-increasing crime rate, but it is only too easy to imagine “It will never happen to me.” Unfortunately, statistics show that it really can happen to you and, if

如图所示,用10牛力沿斜面拉A物,则A物沿倾角为30°的MN斜面匀速上升;若用2牛力沿MN斜面上推A物,则A物匀速下滑.试问如把MN水平放置后,要使A在MN上匀速前进需加多大的水平力?

如图所示,用10牛力沿斜面拉A物,则A物沿倾角为30°的MN斜面匀速上升;若用2牛力沿MN斜面上推A物,则A物匀速下滑.试问如把MN水平放置后,要使A在MN上匀速前进需加多大的水平力?