If the system composed of man and swing is regarded as a pendulum, the cycloid is fixed at point O, and the expansion and contraction of the cycloid itself and the mass of the cycloid are ignored. Imagine that people squat quickly at the maximum deflection angle and stand quickly at the lowest point. The squatting and standing processes are completed in an instant. The squatting and standing of the human body lead to the rise and fall of the center of mass of the system, which is equivalent to the change of effective swing length. In this way, we can abstract the system composed of human and swing into a variable length original pendulum, which is called the variable length original pendulum model.

Swing is a kind of sports and entertainment. Because of its simple equipment and easy learning, it is deeply loved by people and soon became popular everywhere. With the rise of various modern sports events, swing has become a special activity for children in most areas of China, except that it is still popular in a few areas.