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如图所示，MN、PQ为水平放置、相距为d的两平行金属板，两板间电压为U，且上板带正电，板MN中央有一个小孔O，板间电场可认为匀强电场。AB是一根长为L（L<d）、质量为m的均匀带负电的绝缘细杆。现将杆下端置于O处，然后将杆由静止释放，杆运动过程中始终保持竖直。当杆下落L/3时速度达到最大。重力加速度为g。求：

（1）细杆带电荷量；

（2）杆下落的最大速度；

（3）在杆下落过程中，杆克服电场力做功的最大值。

答案

（1）（2）（3）

（1）设杆的带电量为q，板间电场强度为E。杆下落L/3时，a=0，则

     两式解得

（2）杆下落L/3时，速度最大，由动能定理得

   联立②、④式解得     

（3）当杆下落速度为零时，杆克服电场力做功最大，设此时杆下落距离为h，则

解得  杆克服电场力做功    

单项选择题

套管成孔灌注桩复打法施工的作用是()。

A.节约混凝土用量 

B.增大入土深度 

C.提高桩的承载力 

D.加快打桩速度
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单项选择题

Even plants can run a fever, especially when they are under attack by insects or disease. But (1) humans, plants can have their temperature (2) from 3,000 feet away—straight up. A decade ago, (3) the infrared scanning technology developed for military purpose and other satellites, physicist Stephen Paley (4) a quick way to take the temperature of crops to determine (5) ones are under stress. The goal was to let farmer (6) target pesticide spraying (7) rain poison on a whole field, which (8) include plants that don’t have the pest problem.

Even better, Paley’s Remote Scanning Services Company could detect crop problem before they became (9) to the eye. Mounted on a plane flown at 3,000 feet (10) , an infrared scanner measured the heat emitted by crops. The data were (11) into a color-coded map showing (12) plants were running "fevers". Farmers could then spot spray, using 50 to 70 percent less pesticide than they (13) would.

The bad news is that Paley’s company closed down in 1984, after only three years. Farmers (14) the new technology and long-term backers were hard (15) . But with the renewed concern about pesticides on produce, and refinements in infrared scanning, Paley hopes to (16) into operation. Agriculture experts have no doubt about the technology works. "This technique can be used (17) 75 percent of agricultural land in the United States," says George Oerther of Texas A＆M. Ray Jackson, who recently retired from the Department of Agriculture, thinks (18) infrared crop scanning could be adopted by the end of the decade. But (19) Paley finds the financial backing (20) he failed to obtain 10 years ago.

11（）

A.transmitted

B.transferred

C.transformed

D.transported
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