— _____________ is Beijing?
— It's in the north of China.[ ]
A. What
B. Who
C. Where
答案:C
走行于桡神经沟内的动脉是()
A.桡动脉
B.腋动脉
C.旋肱后动脉
D.肱深动脉
E.旋肱前动脉
(8分)如图所示,绝缘细线长为L,最大张力为T,一端固定于O点,另一端连接一带电荷量为q,质量为m的带正电小球,若T>mg,求:
(1)若要小球保持静止,可在O点放置一带电量为多少的点电荷;
(2)若要小球静止时细线与竖直方向成α角,需叠加匀强电场的最小电场强度E为多大.