滑块以速率v1靠惯性沿固定斜面由底端向上运动,当它回到出发点时速率变为v2，且v2＜v1.若滑块向上运动的位移中点为A，取斜面底端重力势能为零，则( )
A．上升时机械能减少,下降时机械能增加
B．上升时机械能增加,下降时机械能减少
C．上升过程中动能和势能相等的位置在A点上方
D．上升过程中动能和势能相等的位置在A点下方
答案：C
由v2<v1可知，斜面与滑块间有摩擦，无论上升还是下降时，都有机械能损失，故AB错.设上升时最大高度为H，最大位移为Ek=Ep时的高度为h，则
2mgh+Fμ·
·L=mgH+Fμ·L，可得
h=
·
，则该点位于A点上方，故选项C正确.
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| A driver 1 his car by the road to have a rest. He closed his eyes and fell asleep. A man came up and knocked at his window to ask the time. The driver 2 his eyes and looked 3 his watch. "It's 8:05." he said. Then he closed his eyes again. Soon another man knocked at the window and asked, "Sir, 4 you have the time." The driver looked at the watch and told 5 man it was 8:25. He couldn't have a good rest, so he wrote some words on a piece of paper and put 6 on the window. It said,"I have no watch. "Then he closed his eyes and fell asleep again. 7 later a third man 8 along and saw the paper. He began knocking at the 9 . "Hi, sir." he said, "It's 10 now." | ||||
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