题干

如图所示,在O点放一个正电荷,在过O点的竖直平面内的A点,自由释放一个带正电的小球,小球的质量为m、电荷量为q.小球落下的轨迹如图中虚线所示,它与以O为圆心、R为半径的圆(图中实线表示)相交于B、C两点,O、C在同一水平线上,∠BOC=30°,A距离OC的竖直高度为h.若小球通过B点的速度为v,试求:

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    There are many differences between country life and city life in many ways. One person even does different things in the two places.

    Julia lived in the countryside, but one year she decided to visit the capital city to do some shopping and to see the sights. She stayed at a hotel near the central market. She had seldom been to the city before, and was very excited about what she would find.

    On the first morning of her visit, as she walked from the hotel to the market, she passed a beggar(乞丐). He was holding up a notice, which said, “Blind from birth. Please give generously(慷慨地).”

    Julia felt sorry for the blind beggar and she bent down and put a dollar coin into his bowl. “Thank you.” he said.

    The same thing happened again the following day.

    On the third day, however, Julia did not have a dollar coin. She had only fifty cents, so she dropped this into the beggar's bowl.

    “What have I done wrong?” the beggar said, “Why are you so stingy today? ”

    Julia was very surprised at what the beggar said. “How do you know I haven't given you a dollar?” she said, “If you're blind you can't know what coin I put into your bowl.”

    “Ah, ” explained the beggar, “The truth is that I'm not blind. I'm just looking after this place for the regular beggar while he's on holiday.”

    “On holiday?” Julia said, “And what exactly does your blind friend do on holiday?”

    “He goes into the countryside,” the man said, “and takes photographs. He's a very good photographer.”