在中学物理中,有一类三力动态平衡问题,即一力确定,另两力有一力或两力大小恒定或夹角不变.此类问题用常规方法难以解决,若巧画圆来解题,则能使问题直观明了.运用此方法的关键是巧画圆构建力的矢量三角形,再根据三角形的变化得出问题的正确答案. In middle school physics, there is a class of three dynamic balance problem, that is, a force to determine the other two forces have a force or both the same size or constant angle. Such problems with the conventional method is difficult to solve, , You can make the problem intuitive and clear.The key to using this method is to draw the vector triangle of the circular constructional force and then to get the correct answer to the problem according to the change of the triangle.