15. 如图所示,空间中存在电场强度
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的水平向右的匀强电场,在电场中有轨道
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。其中
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部分为竖直平面内的轨道,
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为水平面内的轨道。AB段是倾角
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的倾斜直轨道,长为
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, 与竖直平面内圆轨道1相切于B点,圆轨道1的半径
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, C、
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为轨道最低点且略微错开,
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为水平直轨道,长度为
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, DE为半圆形水平弯道,其半径
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, EF也为水平直轨道,长度与
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相同。A点固定一轻弹簧(弹簧长度相对于L不计),一绝缘带正电小球在外力作用下先挤压弹簧,然后由静止释放,弹簧将其弹性势能
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全部转化成小球的动能。已知小球的质量
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, 电荷量
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, 小球与
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、EF的动摩擦因数为0.75,其他轨道对小球的阻力可忽略不计,重力加速度
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。
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。求:
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