一物体质量为
，放在斜面上静止，受一沿斜面向上的力
作用，产生沿斜面向上的加速度
，若将此沿斜面上的力的F的大小变为
，其他条件不变，则物体加速度将
A．大于
B．在与之间
C．等于
D．等于
答案：A
题目分析：当水平拉力变为2F时，物体所受的滑动摩擦力大小没有变化，根据牛顿第二定律对前后两种进行研究，分析两次加速度之比，进行选择．解答：解：设物体所受的滑动摩擦力大小为f，当拉力变为2F时物体的加速度为a′．根据牛顿第二定律得
F-f=ma
2F-f=ma′
则
，即a′＞3a
故选A。
点评：本题考查对牛顿第二定律的理解能力，公式F=ma中F是合力，不是拉力，不能简单认为拉力变为3倍，加速度就变3倍．
| 完形填空。 | |||
| My dearest Aunt Mei lives in the __1__. She grows __2__ there and sells them in town. Sometimes she can't sell out all the vegetables, __3__ she brings (带来) some to us. Aunt Mei is old and short, but she's very __4__. She can walk __5__. Aunt Mei has no__6__ and she lives with her only white cat. Every summer, I go to stay with her for a week or two. She often __7__ me to a farm nearby (在附近). Life in the country is interesting. I can go fishing and I can __8__ birds singing. But Aunt Mei doesn't __9__ me swim. She says it's dangerous. Now the farmers (农民) don't do farm work __10__ animals. They use tractors. All of them look happy. | |||
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