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【简答题】
(2019·全国卷I,25)竖直面内一倾斜轨道与一足够长的水平轨道通过一小段光滑圆弧平滑连接,小物块B静止于水平轨道的最左端,如图(a)所示。t=0时刻,小物块A在倾斜轨道上从静止开始下滑,一段时间后与B发生弹性碰撞(碰撞时间极短);当A返回到倾斜轨道上的P点(图中未标出)时,速度减为0,此时对其施加一外力,使其在倾斜轨道上保持静止。物块A运动的v-t图象如图(b)所示,图中的v 1 和t 1 均为未知量。已知A的质量为m,初始时A与B的高度差为H,重力加速度大小为g,不计空气阻力。 (1)求物块B的质量; (2)在图(b)所描述的整个运动过程中,求物块A克服摩擦力所做的功; (3)已知两物块与轨道间的动摩擦因数均相等。在物块B停止运动后,改变物块与轨道间的动摩擦因数,然后将A从P点释放,一段时间后A刚好能与B再次碰上。求改变前后动摩擦因数的比值。
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