一个初动能为E的小物块从斜面底端冲上足够长的斜面，返回斜面底端时速度大小为v，该过程物体克服摩擦力做功为
．若小物块冲上斜面的初动能变为2E，则（ ）E 2
A．返回斜面底端时动能为3E 4
B．返回斜面底端时动能为3E 2
C．返回斜面底端时速度大小为2v
D．返回斜面底端时速度大小为2v
以初动能为E冲上斜面并返回的整个过程中运用动能定理得：
mV2-E=-1 2
①E 2
设以初动能为E冲上斜面的初速度为V0，则以初动能为2E冲上斜面时，初速度为
V0，加速度相同，根据2ax=V2-V02可知第二次冲上斜面的位移是第一次的两倍，所以上升过程中克服摩擦力做功是第一次的两倍，整个上升返回过程中克服摩擦力做功是第一次的两倍，即为E．2
以初动能为2E冲上斜面并返回的整个过程中运用动能定理得：
mV′2-2E=-E ②1 2
所以返回斜面底端时的动能为E，A错误，B错误；
由①②得：V′=
V，C错误，D正确．2
故选D．
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| A large cat arrived at a farm where many mice lived.None of them dared go outside for fear of being 1 by the cat. The mice decided to hold a 2 to find a way of stopping themselves from being in_3_.One day, when the cat was out, mice of all ages 4 into the meeting room. "Let's build a big trap, "one mouse 5 .When this idea was 6 , another said, "What about poisoning (毒死) her?"But 7 knew where to buy the poison.One young mouse, whose mother had become the cat's food, 8 said, "Let's cut her teeth, so she can do no more harm."But the meeting did not accept his 9 . At last, one mouse, 10 than the rest, jumped onto a table.Waving a bell, he called for 11 , "Be quiet!We'll tie this bell to the cat's tail, so we'll always know 12 she is!We'll have 13 to escape, and the slower and weaker mice will hear her coming and be able to 14 !" Hearing the words, everyone got 15 and they congratulated him on his good idea. "We'll tie it so tightly (紧紧地) that it will never 16 !" "She'll never be able to walk 17 again!Why, the other day, she suddenly appeared right in front of me!Just imagine..." However, the wise mouse said, "We must 18 who is going to tie it on her tail." There was not a sound in the room.Nobody was 19 enough to put the plan into action.The meeting ended without any decision being made. It's often very 20 to have bright ideas, but putting them into practice is a more difficult matter. | ||||
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