如图所示是校长为新装的空调室外设计的支架，支架可以看做杠杆，以O为支点，在支架P孔处拉一根铁线到墙上固定，要使铁线受到的拉力最小，请从P孔处画出铁线到墙的连线，并画出铁线拉力F的力臂l．
如图所示：
题目分析：由杠杆平衡的条件F1×L1="F" 2×L2可知，在阻力、阻力臂一定的情况下，要使所用的动力最小，必须使动力臂最长即为OP的长度，由图示可知，连接OP，即为动力臂L1，过P点做垂直于OP的作用力F，如图所示。
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| When my sister and I were growing up, we used to spend summers with my grandparents. Every morning we had to 1 early and help them with the 2 chores (零活). We helped my grandfather 3 the animals and we helped my grandmother in the garden. Then we'd have a big, delicious 4 . We loved the 5 of the hay (干草) in the barn (厩), and we used to 6 there a lot. Once I got hurt when I fell from the hayloft (干草棚), and my grandparents had to take me to the 7 . They were very worried about it and were 8 to let us out of their sight (视野) after that. I can still hear my grandfather's voice, "Now, be 9 in the barn, you two." Now summers come and go, but every summer I remember those 10 times at my grandparents. |
| ( )1. A. sleep ( )2. A. factory ( )3. A. feed ( )4. A. breakfast ( )5. A. shadow ( )6. A. exercise ( )7. A. doctor ( )8. A. peaceful ( )9. A. favorite ( )10. A. helpful | B. get up B. garden B. run B. meat B. form B. play B. parents B. polite B. excited B. free | C. stand up C. farm C. drink C. lunch C. warmth C. fall C. teacher C. easy C. quiet C. noisy | D. act D. house D. wash D. diet D. smell D. raise D. house D. afraid D. careful D. wonderful |