质量为m的物体在拉力F的作用下从静止以2g的加速度匀加速上升h，不计空气阻力，则
A．拉力F做功2mgh
B．物体克服重力做功2mgh
C．物体的动能增加了2mgh
D．物体的机械能增加了2mgh
答案：C
根据牛顿第二运动定律可得
所以
，故拉力F做功为3mgh，A错误。重力做功为mgh，所以物体克服重力做功为mgh,B错误。拉力做的功一部分转化为动能，一部分转化为重力势能，所以物体的动能增加了2mgh，物体机械能增加了3mgh，C正确，D错误。
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根据对话内容，用方框中所给词的适当形式填空。
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| A:Do you have any 1 ? B:Yes.l hope l will be an animal doctor when I grow up. A:Why? B:Because I like animals very much and want to take care of 2 ． A:That sounds 3 ． B:What about you? A:I 4 to be a train driver. I like traveling very much. B:That way you will be able to travel for free. A:Yes.1 wish I 5 visit different places and make lots of friends. |