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一、利用图象法解题可以清楚地描绘出题目的物理过程:例如,用细绳 OA、OB 悬挂着一个重物[如图1(α)]在保持重物位置不动的前题下,使绳的 B 端沿半径等于绳长的圆周轨道向 C 点移动,在移动过程中,绳 OB 上的张力大小如何变化?这道题用力示图方法求解比较直观、明瞭。方法是根据物体的重力 mg 大小和方向不变,AO 绳的张力方向不变,因此可作力示图〔如图1(b)〕,由图可以看出。当 B 端移向 C 点的过程中,绳 OB 的张力大小是先减小(B→G),因为在△DBG 中斜边 DB 大于直角边 DG,在 G 点最小。而后再增大(G→C),并由△DBC 中得DB=DC,即绳 OB 在 B 点时的张力等于在 C 点时的张
First, the use of image method to solve the problem can clearly describe the physical process of the title: for example, with a string OA, OB hanging a weight [Figure 1 (α)] under the premise of maintaining the position of the weight is not moving Let the B end of the rope move to point C along a circular orbit whose radius is equal to the length of the rope. How does the tension on the rope OB change during the movement? The solution to this question is intuitive and clear. The method is based on the weight and direction of the object’s gravitational force, and the direction of the tension of the AO rope is not changed. Therefore, the force diagram can be displayed as shown in Figure 1(b). It can be seen from the figure. When B moves to point C, the tension of the rope OB first decreases (B→G) because the DB of the hypotenuse DB is greater than the right-angled edge DG in ΔDBG, and is smallest at the point G. Then increase (G→C) and DB=DC from △DBC, that is, the tension of rope OB at point B is equal to the tension at point C.