物体同时受到同一平面内的三个力作用，下列几组力中，它们的合力不可能为零的是
A．5N 、 7N 、 8 N
B．2N 、 3N 、 5N
C．1N 、 5N 、 10 N
D．1N 、 10N 、 10N
答案：C
题目分析：三个力的合力为零时，则第三个力肯定在第一二两个力的合力范围里，由此判断选项即可．
三个力合力为0时，则任意两个力的合力与第三个力大小相等，方向相反，由此可知，任意一个力在另外两个力的合力范围．
A、5N和7N的合力范围为：2N-9N，8N在合力范围里，故三个力的合力可能为0；
B、2N和3N的合力范围为：2N-5N，5N在合力范围里，故三个力的合力可能为0；
C、1N和5N的合力范围为：4N-5N，10N不在合力范围里，故三个力的合力不可能为0；
D、1N和10N的合力范围为：9N-11N，10N在合力的范围里，故三个力的合力可能为0．
故选C．
点评：熟悉三个力合力最小值的判定方法，熟悉平衡条件的推论即其应用．
| The question of what children learn and how they should learn it is continually being debated and redebated. Nobody dares any longer to defend the old system, the learning of lessons parrot-fashion and the grammar with-a-whip system, which was good enough for our grandparents. The theorists of modem psychology have stepped in to argue that we must understand the needs of children. Children are not just small adults; they are children who must be respected as such. Well, you may say, this is as it should be, and a good idea. But think further. What happens "Education" becomes the responsibility not of teachers, but of psychologists. What happens then Teachers worry too much about the psychological implications of their lessons, and forget about the subjects themselves. If a child dislikes a lesson, the teacher feels that it is his fault, not the child’s. So teachers worry whether history is "relevant" to modem young children. And do they dare to recount stories about violent battles Or will this make the children themselves violent Can they tell their classes about children of different races, or will this encourage racial hatred Why teach children to write grammatical sentences Verbal expression is better. Sums Arithmetic No, no; real-life mathematical situations are more understandable. You see you can go too far. Influenced by educational theorists, who have nothing better to do than write books about their ideas, teachers leave their teacher-training colleges filled with grand, psychological ideas about children and their needs. They make elaborate, sophisticated preparations and try out their "modem methods" on the long-suffering children. Since one "modem method" rapidly replaces another, the poor kids will have had a good bellyful by the time they leave school. Frequently the modem methods are so sophisticated that they fail to be understood by the teachers, let alone the children; even more often, the relaxed discipline, so essential for the "informal" feeling the class must have, prevents all but a handful of children from learning anything. |
Grammatical sentences are regarded as unimportant because ______.
A．words are uttered out of natural feelings only
B．it is better to use verbs only
C．it is felt that formal grammar rules might cause unnatural expression
D．talking freely and naturally with no regard to grammatical rules is a better form of expression