物体A和物体B叠放在光滑水平面上静止，如图所示。已知mA="4" kg, mB="10" kg, A, B间的最大静摩擦力fm="20" N。现用一水平向右的拉力F作用在A上，则
A．当F="20" N时，A对B的摩擦力大小是20N
B.当F<20 N时，A、B都静止
C.只要F>20 N, A、B 间就发生相对运动
D.只要F>28 N, A、B间就发生相对运动
答案：D
题目分析：假设AB相对静止，则整体分析有
，单独分析B则有静摩擦力
，即
，带入整体受力的方程则有
，即当时二者保持相对静止，但不是静止答案B错。当
时二者相对滑动，对照答案BD对C错。当F="20" N时AB一起匀加速运动，A不可能合力为0，即摩擦力不可能等于20N答案A错。
| 完形填空。 | ||||
| One day, a young man was cleaning out his late grandfather's belongings when he came across a bright red envelope. Written on the 1 were the words, "To my grandson." 2 it was his grandfather's handwriting, the boy opened the envelope. A letter inside read: Dear Ronny, Years ago, you 3 to me for help. You said, "Grandpa, how is it that you've 4 so much in your life? You're still full of 5 , but I'm already tired of struggling. How can I get that same enthusiasm that you've got?" I didn't know what to say to you then. But knowing my days are 6 , I figure that I 7 you an answer. So here is what I believe. I think it has something to 8 with how a person looks at things. I call it "keeping your eyes wide open". First, realize that life is filled with surprises, but many are good ones. If you don't keep 9 for them, you'll miss half the excitement. Expect to be excited 10 , and you will be. When you meet up with 11 , welcome them. They'll leave you wiser, stronger, and more capable than you were the day before. When you make a mistake, be 12 for the things it taught you. Learn to use that lesson to help you reach your goals. It's also important to 13 exactly what you want. Then keep your mind focused on it, and be prepared to receive it. As you grow with the years, you'll be given bigger shoes to fill. So be ready for endings as well as challenging 14 . Sometimes we have to be brave enough to move from the familiar to the unfamiliar. Life isn't just reaching peaks. Part of it is moving from one 15 to the next. If you 16 too long in between, you might be tempted to 17 . So consider all the pathways ahead, and decide which ones to 18 . Then believe in yourself, get up, and get going. Most important of all, never give up on yourself. The person that ends up a winner is the one who resolves (决定) to 19 . Give life everything you've got, and life will give its best 20 to you. | ||||
|