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【原题再现】(2013年全国新课标I 24)(13分)水平桌面上有两个玩具车A和B,两者用一轻质细橡皮筋相连,在橡皮筋上有一红色标记R。在初始时橡皮筋处于拉直状态,A、B和R分别位于直角坐标系中的(0,2l)(0,-l,)和(0,0)点。已知A从静止开始沿y轴正向做加速度大小为a的匀加速运动;B平行于x轴朝x轴正向匀速运动。在两车此后运动的过程中,标记R在某时刻通过点(l,l)。假定橡皮筋的伸长是均匀的,求B运动速度的大小。
【Reproduction of the original title】 (2013 National New Course Standard I 24) (13 points) Horizontal There are two toy cars A and B on the tabletop, which are connected by a light and fine rubber band with a red mark R . The rubber band is initially straightened and A, B, and R are (0,2l) (0, -l,) and (0,0) points in Cartesian coordinates, respectively. It is known that A makes a uniform acceleration along the y-axis in the positive direction from rest when the acceleration is a; B moves in a positive direction toward the x-axis in a direction parallel to the x-axis. During the subsequent movement of the two vehicles, the marker R passes the point (l, l) at a certain moment. Assuming the elongation of the rubber band is uniform, find the size of the B movement speed.