将四块相同的坚固石块垒成圆弧形的石拱，其中第3、4块固定在地基上，第1、2块间的接触面是竖起的，每块石块的两个侧面间所夹的圆心角为30°，假设石块间的摩擦力可以忽略不计，则第l、2块石块间的作用力与第1、3块石块间的作用力二大小之比值为
A．
B．
c．
D．
答案：B
如图对第一个石块进行受力分析，由几何关系知：θ=600，所以有N12 ：N13=sin60°=
；
故选B．
| 完型填空 | |||
| I spent two months with an American family last year when I studied in America. It is said, "The best house is in the U.S.A., the best wife is in Japan, and the best food is in China. "So I took __1__ and pleasure to __2__ "the best food" for my American family. Each day, regardless of my poor skill of cooking,I performed(表演)my play in __3__ way:changing the dishes' colour,the meat's type or the soup's style(风味)．Thus,my American friends could discover the __4__ of eating Chinese food． Because of my"__5__" job,I was often rewarded(奖励)by their __6__ of appreciation (感激) such as "very tasty","excellent", etc. But the joke was here: yesterday, I cooked Sichuan－style __7__ for dinner. I was __8__ at my achievement(成就)． When all the family members arrived at the table,they first looked __9__ at the fish,then looked at me. "Why do all the fish have their __10__?" they asked,"It's horrible!" Horrible or beautiful? I don't know. But I __11__ know that fish heads are delicious. The __12__is the best part of fish; in __13__ only the respectable(受敬重的)guest can have the special __14__of enjoying it. Another __15__thing is:the American friends often said to me" __16__ "as they __17__ fruits or other things. According to Chinese tradition(传统),I would simply smile in answer to their kindness __18__ actually helping myself to any of the food. __19__I said "help yourself" to them whenever I brought home fruits. Dear me,no matter how much or how often I brought home food, as long as it was nice to their taste,they __20__ hesitated(犹豫) to use their hands to "help themselves" until all was finished. | |||
| ( )1. A. worry ( )2. A. buy ( )3. A. their ( )4. A. way ( )5. A. hard ( )6. A. expressions ( )7. A. meat ( )8. A. praised ( )9. A. pleasedly ( )10. A. bones ( )11. A. do ( )12. A. tail ( )13. A. fact ( )14. A. honour ( )15. A. real ( )16. A. please help us ( )17. A. brought home ( )18. A. not ( )19. A. At last ( )20. A. usual | B. attention B. fetch B. a new B. joy B. daily B. sentence B. fish B. pleased B. excitedly B. flesh(肉) B. will B. head B. China B. seat B. exciting B. excuse me B. wanted B. but not B. Above all B. often | C. delight C. cook C. the same C. trouble C. good C. doing C. soup C. bad C. surprisedly C. tails C. want to C. soup C. USA C. table C. interesting C. help yourself C. were buying C. without C. After all C. never | D. pride(自豪) D. leave D. a simple D. importance D. new D. smile D. egg D. surprise D. worriedly D. heads D. should D. flesh D. the West D. plate D. easy D. eat them please D. were eating D. nor D. In return D. had |