题干

如图所示,电子以速度v0沿与电场垂直的方向从A点飞入匀强电场,并且从另一侧的B点沿与电场成150°角的方向飞出,已知电子的质量为m,电荷量为e,求A、B两点的电势差.

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答案(点此获取答案解析)

解:电子垂直进入电场做类平抛运动,在水平方向上做匀速直线运动,在竖直方向上做匀加速直线运动,根据平行四边形定则知,B点的速度为:

vB= v0sin30°<

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阅读理解

    It's generally agreed that heroes are great and always admired by common people. However, most of us have no clear idea about heroes.At some point,we all wonder if we need a hero and what a hero really is.

    Although there are a lot of differences in cultures,heroes around the world generally share a number of characteristics(特点)which give us courage and make us want to learn from them.A hero does something worth talking about.A hero usually put others before and does his/her best to help people in need.A hero has a special story to tell and is thought highly of by people.

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    A hero has a vision(视野)from the mountaintop.He/She has the power to move people.He/She creates new possibilities.Without Gandhi,India might still be part of Britain.Without Martin Luther King,Jr.,Americans might take different buses,eat in different restaurants,go for walks in different parks,and shop in different stores because of the different colors of their skin.

    There might be changes in society without a hero, but the speed of change would be rather slow.Thanks to heroes in history,they make the society develop so rapidly.