如图，水平面上有一重为40N的物体，受到F1＝13N和F2＝6N的水平力作用而保持静止。已知物体与水平面间的动摩擦因数μ＝0.2。设最大静摩擦力等于滑动摩擦力，求：
（1）物体所受的摩擦力的大小与方向？
（2）若将力F1撤去，物体受到的摩擦力的大小和方向如何？
（3）若撤去的力不是F1而是F2，则物体受到的摩擦力大小方向又如何？
解：（1）fmax=8N
F1-F2=7N< fmax
故f=7N
（2）将F1撤出后F2=6N< fmax
故f=6N
（3）将F2撤出后F1=13N>fmax
故f= fmax=8N。
| Children are a relatively modem invention. Until a few hundred years ago they look like adult, wearing grown-up clothes and grown-up expressions, performing grown-up tasks. Children did not exist because the family as we know it had not evolved. Children today not only exist; they have taken over in no place more than in America, and at no time more than now. It is always Kids Country here. Our civilization is child-centered, child-obsessed. A kid’s body is our physical ideal. In Kids Country we do not permit middle-aged. Thirty is promoted over 50, but 30 knows that soon his time to be overtaken will come. We are the first society in which parents expect to learn from their children. Such a topsy-turvy situation has come to abort at least in part because, unlike the rest of the world, ours is an immigrant society, and for immigrants the only hope is in the kids. In the Old Country, that is, Europe, hope was in the father, and how much wealth he could accumulate and pass along to his children. In the growth pattern of America and its ever- expanding frontier, the young man was ever advised to GO WEST; the father was ever inheriting from his son. Kids Country may be the inevitable result. Kids Country is not at all bad. America is the greatest country in the world to grow up in because it is Kids Country. We not only wear kids clothes and eat kids food; we dream kids dreams and make them come tree. It was, after all, a boys’ game to go to the moon. If in the old days children did not exist, it seems equally true today that adults, as a class, have begun to disappear, condemning all of us to remain boys and girls forever, jogging and doing push-ups against eternity. |
By saying "condemning all of us to remain boys and girls forever, jogging and doing push-ups against eternity", the author means that ______ .
A．she thinks people shouldn’t be so concerned about physical fitness
B．she feels too old and tired to do such hard exercise
C．American society is overemphasizing youth and physical appearance
D．what happened to children centuries ago may occur to adults in America soon