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题1如图1所示,O为杠杆的支点,为了提升重物A,用一个跟杠杆始终保持垂直的力F,使杠杆由竖直位置转到水平位置,在这一过程中()(A)杠杆始终是省力的.(B)杠杆始终是费力的.(C)杠杆先是省力后是费力.(D)杠杆先是费力后是省力.(2001年上海初物竞)
Question 1 is shown in Fig. 1. O is the fulcrum of the lever. To lift the weight A, use a force F that keeps vertical with the lever and turn the lever from the vertical position to the horizontal position. In this process () ( A) Leverage is always labor-saving. (B) Leverage is always laborious. (C) Leverage is labor-intensive after labor-saving. (D) Leverage is labor-saving and effort-saving first. (2001 Shanghai Bidding)