How can the swing swing higher and higher without anyone pushing or pulling?

Review expert: Luo Huiqian (Associate Researcher, Institute of Physics, Chinese Academy of Sciences)

Speaking of swings, I believe everyone must be familiar with them. Swings carry the joy of our growth and always bring back our good memories. With the help of a piece of wood and two ropes, you can swing in the air. However, do you know what scientific principles allow the swing to carry us higher and higher when we swing?

The origin of the swing

The history of swinging in China is quite long. In ancient times, our primate ancestors climbed trees or crossed gullies while climbing or running in order to pick wild fruits, escape or hunt wild animals. Out of instinct, they mechanically grabbed vines with their arms and relied on the swing of vines to obtain food from high places, which gave the swing a primitive prototype.

Artifacts from the end of the Minoan civilization, showing people swinging on a swing

Source: Wikipedia

After a long period of evolution, in the era of human civilization, early swings appeared in many places, which is reflected in many archaeological studies, such as the Minoan civilization in Greece and the Shanrong people in the north of my country during the Spring and Autumn Period. Swinging gradually became a form of entertainment, and there were more and more styles. The sport of swinging was widely spread and gradually became popular throughout the country, and was welcomed by the general public.

Source: Movie Stills

During the Tang and Song dynasties, people's living standards gradually improved, and swinging became a festival carnival and entertainment medium. Qingming Festival is also known as the "Swing Festival", and some legends believe that swinging can drive away all diseases.

During the Qingming Festival, people also go outings and build swings with bamboo to dance and play. Today, swings have a history of thousands of years in China. They have witnessed the rise and fall of dynasties and left a national imprint. Swings have developed into an art form that combines function and entertainment, and is endowed with a lot of beauty, which is pleasing to the eye and interesting at the same time.

The swing is a physical phenomenon

A swing can actually be considered a simple mechanical system. It is subject to two external forces: one is the vertical downward gravity; the other is the constraint force along the direction of the swing rope. The simplest way to play a swing is to sit on the swing board and wait for someone to push you hard. The movement of the swing at this time is similar to that of a simple pendulum, but because air resistance and friction resistance will hinder the swing, the swing amplitude will become smaller and smaller until it stops.

If a person applies external force to push the swing continuously, it will make the swing swing higher and higher. When the swing reaches the highest point, applying an external force in the direction of the swing will push the swing higher; but if a force is applied in the opposite direction of the swing's movement, the external force will hinder the swing's movement and reduce its height.

When we swing on a swing, we will feel a brief sense of weightlessness when we swing downwards, and we may feel nervous and fearful. When the swing swings higher and higher and quickly swings downwards, the human body is affected by the centrifugal force, and the lower abdomen feels slightly compressed, and there is a feeling of wanting to urinate. Overcoming tension and fear while swinging can enhance self-control and is beneficial to physical and mental health. In addition, people who often swing rarely feel dizzy when riding in a car or boat.

How to swing higher and higher?

How to keep the swing swinging and swinging higher without relying on external force from the ground is the goal that everyone who participates in swing sports hopes to achieve.

When we observe people swinging on a swing, we can find that those who swing higher and higher always stand up or squat. Although there is no external force pushing them, the actions taken by people actually increase the energy of the system. Therefore, when considering resistance, as long as the energy converted into mechanical energy by human body movements is greater than the energy consumed by overcoming resistance, the swing will swing higher and higher.

Source: Baidu Encyclopedia

The center of gravity of the human body and the swing system can be simplified as a mass point. The center of gravity position changes with the change of posture. When you bend your knees and squat, the center of gravity is lower, while when you stretch your legs and stand up, the center of gravity is higher. The swing is at a high place. If the person is standing, the center of gravity of the system is the highest, the gravitational potential energy is the largest, and the kinetic energy is the smallest.

When the swing swings back from a high place, the person starts to squat, the center of gravity moves downward, and the gravitational potential energy of the whole system is constantly converted into kinetic energy, and the speed is getting faster and faster, until the swing returns to the lowest point, the speed is the fastest, the kinetic energy of the system reaches the maximum, and the gravitational potential energy is the minimum. If the person does not squat during this process, the kinetic energy of the system will not increase that much, but will be continuously lost due to resistance.

When the swing swings up from the lowest point, the person starts to stand up quickly, which raises the center of gravity, increases the potential energy, and slows down the speed of kinetic energy reduction. The total mechanical energy of the system actually increases, and the swing will eventually swing higher than the last time. This is because the human body consumes its own energy to overcome gravity and do work, so it will feel that the legs and feet are very hard, and it will feel very tired after a long time.

As shown in the figure below, the swinging process is regarded as a simple pendulum motion, the red area is the first half, and the blue area is the second half. By changing its own state so that the swing angle in the second half is greater than that in the first half, it can swing higher and higher. The energy comes from the work done when retracting and releasing the legs.

Simple diagram of the swinging process Source: Internet

From this, it can be seen that swinging on a swing also requires a certain technique. When the swing is swinging down, bend your knees and squat to lower your center of gravity, and when it is swinging up, stand up with your legs to raise your center of gravity, so that the swing can swing higher and higher. When swinging on a swing, you need to hold the rope with both hands and exert force as you swing up and down. The whole process mainly relies on the strength of your waist, legs and hips to achieve up and down swings.

Swings have a long history. Although they are simple in structure, they contain a wealth of physical knowledge. In Chinese culture, swings also represent the beautiful meaning of health and happiness. Let us swing on the swings again after studying and working, swinging away our worries and bringing back laughter.