题目内容
【题目】判断下列情况下物体的动能、重力势能、机械能的变化情况。（选填“增大”或“减小”“不变”）
（l）在平直轨道上，列车从车站启动，动能 ，势能 ，机械能 。
（2）站在匀速下降的电梯中的乘客，动能 ，势能 ，机械能 。
【答案】（1）增大；不变；增大．（2）不变；减小；减小
【解析】
试题分析：动能与物体的质量和运动速度有关，质量越大，速度越快，动能就越大．
重力势能与物体的质量和高度有关，质量越大，高度越高，重力势能就越大．
机械能是动能和势能的总和．
列车从车站启动，速度增大，故动能增大；列车在平直轨道上，高度不变，故重力势能不变；动能增加，重力势能不变，故其总和机械能也在增大．
乘客是匀速下降，速度不变，故动能不变；电梯下降，高度减小，故重力势能减小；动能不变，重力势能减小，故其总和机械能也在减小．
故答案为：（1）增大；不变；增大．（2）不变；减小；减小．
【题目】任务型阅读。
阅读短文，根据短文内容，从短文后的五个选项中选出能填入空白处的最佳选项。每个选项只能用一次。
Careful and careless are as different as fire and water. But it is strange that quite a few scientists have both these qualities(品质). They are both careful and careless.
1
Once Newton invited a friend to dinner at home. When they were ready to eat, Newton left to get a bottle of wine(酒). 2 , Newton still didn’t come back. At last his friend found Newton in his lab. When Newton was going to get the bottle of wine, he suddenly had a new idea for his experiment. So he quite forgot his friend and the dinner. Another time, Newton was leading a horse up a mountain, and he held the rein(缰绳) in his hand. While he was walking along, he kept thinking about problems. 3 , he found that the horse was gone.
Einstein was another example. 4 . While waiting, he became lost in thought. It started to rain. The rain kept on for some time. When he took out a piece of paper to write something down, the paper was wet and then he knew that it was raining. 5 , he again forgot he was standing in the rain.
A. But after he put the paper into his pocket B. But after his friend had waited for a long time C. Once he was waiting for a friend at a bridge D. Newton, the famous English Scientist, was such a person E. When he got to the top of the mountain |