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我在讲完“摩擦力”后,有学生问我一个问题;把一端拴重物的绳子绕在木棍上悬在空中,绕的圈数越多重物越不容易掉下来,是不是绳与木棍之间的摩擦力和绳与木棍间的接触面积有关,面积越大摩擦力越大呢?这使我想起了一个曾经做过的有趣实验:绳子一端拴一玻璃瓶,用木棍挑着绳子靠近瓶的一端(如图1),绳子另一端与竖直方向有一定角度,由静止释放.你会想,瓶子一定“啪”的一声掉在地上粉身碎骨.结果却不是那样,瓶子下落一段距离后绳子在棍上绕了几圈,停住了(如图2).
After I finished “Friction”, some students asked me a question; winding a rope with one heavy object around a stick in the air, the more the number of windings around, the more difficult it is to fall off. The friction between the rope and the stick is related to the contact area between the rope and the stick. The larger the area, the greater the friction. This reminds me of an interesting experiment I once did: A wooden stick holds the end of the rope near the bottle (see Figure 1). The other end of the rope has a certain angle with the vertical direction and is released from the rest. You will think that the bottle must be dropped on the ground with a bang. In other words, after the bottle fell a certain distance, the rope wrapped around the stick a few times and stopped (see Figure 2).