When we are on the swing, if we stand still, the motion of swing is similar to that of pendulum. When the swing swings from the lowest point to the highest point, gravity does negative work, and the kinetic energy of the system is transformed into the potential energy of the system. When the swing swings from the highest point to the lowest point, the potential energy of the system is transformed into the kinetic energy of the system. In the whole process, the mechanical energy is conserved, and the swing will swing with equal amplitude. If you want to make the swing swing higher and higher, you must rely on external forces. If you play the swing yourself, you can't rely on external forces, and the system can't get energy from the outside world. Only by doing work with the internal force of the rope, can the internal energy of the swing be transformed into the mechanical energy of the system, and the internal force of the swing can only be achieved by standing up or squatting down on the swing. However, when the person swings to the balance position and pulls the rope with both hands, the rope pulls the person with the same amount of reaction force. This force overcomes human gravity to do work, which makes people suddenly stand up. At this time, people's center of gravity will move up. The gravitational potential energy of the system increases. At this time, the tangential velocity does not change, that is, the kinetic energy does not change, and the mechanical energy of the system increases. When the swing slowly squats down from the lowest point to the highest point, gravity no longer does negative work, but almost does no work, and even can do positive work. When people rise to the highest point, they quickly stand up. It increases the gravitational potential energy. When the swing swings from the highest point to the lowest point, slowly squat down to make its center of gravity drop. At this time, the position of the center of gravity drops, and the gravity still does positive work in this process. Then gravity always does positive work in one cycle. In this way, the swing player can convert his internal energy into the mechanical energy of the swing.
