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【简答题】
如图 a 所示, OA 、 OB 、 OC 三结于 O 点,轻绳 OA 与竖直方向的夹角为 37° ,下方轻绳 OC 悬挂质量为 m 1 = 0.4 kg 的沙桶。轻绳 OB 水平, B 端与放置在水平面上的质量为 m 2 = 1.8 kg 的滑块相连,滑块处于静止状态,已知滑块与水平面间的动摩擦因数为 μ = 0.3 , sin37° = 0.6 , cos37° = 0.8 ,重力加速度 g 取 10 m/s 2 ,最大静摩擦力按滑动摩擦力计算。 (1) 求滑块受到的摩擦力大小; (2) 若缓慢往沙桶中添加细沙,要使滑块静止不动,沙桶和沙的总质量不能超过多少? (3) 若移去滑块,保持 O 点位置不变,用手牵引 OB 由水平位置绕 O 点向上顺时针缓慢转动 90° ,如图 b 所示,求此过程中绳 OB 上拉力的最大值 T max 和最小值 T min 。
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