在初中杠杆内容学习中,力对杠杆的作用效果与力臂有关为其重要内容,但其中的力臂作图一直是学生难以掌握的一个知识点。当力的方向改变时,力臂的大小也随之改变;尤其是,力臂随力的方向改变而改变的动态过程成了学生难以突破的难点。若能制作一个当力的方向改变时,就能及时显示出力臂大小情况的演示仪,则能以直观、形象的教学方式帮助学生突破这一难点。为此,笔者根据几何定理“圆的直径 Learners in the junior high school learning content, the effect of leverage on the lever arm and its important content, but the force arm mapping has been a point of knowledge difficult to master. When the direction of the force changes, the size of the arm also changes; in particular, the dynamics of the arm changing with the direction of the force becomes difficult for students to break through. If you can make a change in the direction of the force, you can promptly show the size of the arm of the instrument, you can intuitive, visual teaching methods to help students break through this difficult. To this end, I based on the geometric theorem ”circle diameter