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一、问题如图1所示,质量为 m 小球 B 放在光滑的水平槽内,现有一长为 l 细绳连接另一质量为 m 小球 A,开始细绳处于松驰状态,A 与 B 相距为 l/2,小球 A以 v_0初速在光滑的水平面上向右运动。当 A 运动到图示某一位置时,细绳被拉紧,试求 B 球开始运动时速度大小 v_B。二、几种常见错误解答对于上述问题,在教学中我们发现在师生中有如下二种错解。错解一:当细绳拉紧时,细绳与水平线成30°角,A球与 B 球碰前速度 v_0的切向分量为 v_0/2,碰后 A 球速度变为 v_A,v_A 的切向分量不变为 v_0/2,水平分量为 v_x(见图2)。由于系统水平方向所受合力为零,因而系统水平方向动量守恒。
First, the problem is shown in Fig. 1. The mass m ball B is placed in a smooth horizontal slot. The existing length is l string and the other mass is m ball A. The string starts to relax. A. B is at a distance of l/2, and the ball A moves to the right on a smooth horizontal surface with v_0 muzzle velocity. When A moves to a certain position in the figure, the string is pulled tight. Try to find the speed of the ball B when the ball starts to move. Second, several common mistakes to answer the above questions, we found in the teaching of teachers and students the following two kinds of misunderstandings. Misunderstanding one: When the string is tightened, the string is at an angle of 30° with the horizontal, and the tangential component of the speed v_0 before the collision between the A ball and the B ball is v_0/2. After the collision, the speed of the A ball changes to v_A, the cut of v_A. The component does not change to v_0/2 and the horizontal component is v_x (see Figure 2). As the resultant force in the horizontal direction of the system is zero, the momentum in the horizontal direction of the system is conserved.