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例1.如图所示,甲乙两木块质量分别为 m_1和 m_2,用细绳连在一起,中间有被压缩的轻弹簧,木块乙放在地面上,在把细绳烧断的瞬间,木块甲向上运动的加速度为 a,则此时地面对木块乙的支持力为____。解:把甲乙两木块及弹簧看成一个整体,受力分析如图:取竖直向上方向为正,依据牛顿第二定律,有N-(m_1+m_2)g=m_1a N=(m_1+m_2)g+m_1a所以此时地面对木块乙的支持力为(m_1+m_2)g+m_1a例2.如图,质量为 M 的框架放在水平地面上,一轻质弹簧上端固定在框架上,下端拴一个质量为 m 的小球,当小球上下振动时,框架始终没有跳起。框架对地面压力为零的瞬间,小球加速度大小为[ ]。
Example 1. As shown in the figure, the masses of A and B blocks are m_1 and m_2, respectively, and they are connected together with a string, with a light spring that is compressed in the middle, and the block B is placed on the ground and the string is blown at the moment. The acceleration of the upward movement of the block armor is a, then the support force for the block B on the ground is ____. Solution: Look at the A and B blocks and the spring as a whole. The force analysis is shown in the figure: take the vertical upward direction as positive, according to Newton’s second law, there is N-(m_1+m_2)g=m_1a N=(m_1+ M_2)g+m_1a So at this time, the supportive force of the ground block B is (m_1+m_2)g+m_1a Example 2. As shown in the figure, the frame with the mass M is placed on the horizontal ground, and the upper end of a light spring is fixed on the On the frame, a ball with a mass of m is placed at the lower end. When the ball vibrates up and down, the frame does not jump. When the frame pressure on the ground is zero, the small ball acceleration is [].