As an important ideal model,simple pendulum has become an important teaching content in mechanics and theoretical mechanics courses.Because of the curved motion of the pendulum,people often use Newton's law of motion or the angular momentum theorem to analyze the dynamic equation of a single pendulum in polar coordinates and natural coordinates.Assuming the initial state of the pendulum and the positive direction of the coordinate system casually may result in a negative angle.However,this negative sign hidden in the angle is usually overlooked by beginners and leads to false conclusions.Proper selection of the diagram and the positive direction of coordinate system can avoid the negative angle,so that the dynamic equation of the pendulum can be analyzed quickly and accurately.
关键词
角动量定理/单摆动力学方程/极坐标/自然坐标/坐标系正方向
Key words
angular momentum theorem/simple pendulum dynamic equation/polar coordinates/natural coordinates/positive direction of coordinate system
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