( 15分 ) (2018·全国卷 I )如图所示, abc 是竖直面内的光滑固定轨道 , ab 水平 ,长度为2 R ; bc 是半径为 R 的四分之一圆弧 ,与 ab 相切于 b 点。一质量为 m 的小球 ,始终受到与重力大小相等的水平外力的作用,自 a 点处从静止开始向右运动。重力加速度大小为 g 。小球从 a 点开始运动到其轨迹最高点 ,机械能的增量为( )
A.
2 mgR
B.
4 mgR
C.
5 mgR
D.
6 mgR
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