People are swinging, and the higher the swing, it shows that mechanical energy is not conserved and is constantly increasing. So where does this increase in mechanical energy come from?
When we are
swinging, if people stand still on the swing, the movement of swinging is similar to the movement of a single 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. The process mechanical energy is conserved, and the swing will
swing at a constant amplitude. If you want to make the swing higher and higher, you have to use external force. When you swing on the swing yourself, there is no way to use external force, and the system cannot get energy from the outside world. Only the internal energy of the swinger and the rope can be converted into the mechanical energy of the system, and the internal energy of the person can only be achieved by standing up or squatting down on the
swing. But when a person swings to a balanced position, pull the rope forcefully with both hands, and the rope pulls the person with the same amount of reaction force. This force overcomes the person's gravity to do work and makes the person stand up suddenly, and the person's center of gravity will move upward at this time.The gravitational potential energy of the system increases. At this time, the tangential speed does not change, that is, the kinetic energy does not change, and the mechanical energy of the system increases. When the swing is slowly squatting from the lowest point to the highest point, gravity will do less work on the center of gravity than when standing, reducing the consumption of potential energy. When the person rose to the highest point, he stood up quickly. Increase the gravitational potential energy. When the swing swings from the highest point to the lowest point, squat slowly to lower its center of gravity. At this time, the position of the center of gravity drops. In this process, gravity still does positive work. Then gravity always does positive work in one cycle. In this way, the swinger can convert his own internal energy into the mechanical energy of the swing.
