Textbooks and related popular science books rarely give in-depth explanations of the internal mechanism of buoyancy, and only explain it from the surface phenomenon, and its explanation methods are mainly as follows: due to the characteristics of the liquid, pressure will be generated, and this pressure will increase with the increase of depth, and at the same time, there is pressure in all directions inside the liquid. This means that the lower bottom of the object will experience more upward pressure from the liquid, while the upper bottom will experience less downward pressure from the liquid. This pressure difference is manifested as a vertical upward force, which we call buoyancy.
It is due to the gravitational force between molecules, which should have the same pressure in all directions (as in weightlessness), but because of gravity, the density of the gas or liquid gradually increases in the direction of gravity. The intermolecular gravitational force also increases accordingly, and when an object is placed in it, the pressure below is naturally greater than the pressure above per square meter. So the object has buoyancy. The greater the pressure on the object. That's why the deeper the water, the greater the pressure of the water. Moreover, due to the gravitational pull between the molecules, the water creates a buoyant force on the object. This buoyancy allows the object to float on the surface of the water instead of sinking. That's why we can easily swim or float on the water.
The same principle applies to gases. In the atmosphere near the Earth's surface, the density and pressure of the air decrease with altitude due to gravity. This is also the reason why airplanes need to reach a certain altitude and speed to take off. Because only at a certain altitude and speed can the wings of an aircraft generate enough lift to allow the aircraft to leave the ground and remain in the air.
The above explanation of buoyancy is not to the point, in fact, buoyancy is the result of the balance of suction and repulsion, there are four fundamental forces in the universe, and the combination of these four fundamental forces can evolve many kinds of forces, including: buoyancy, tension, stress and heat, strong and weak forces are the forces of microscopic particles, and the range of action of these two forces is very short. The electromagnetic force and gravitational force are long-range forces that can be infinitely long, but the gravitational force makes the spatial distance of matter shorter to each other, and the mutual repulsion force of the outer electrons of molecules and atoms will appear when they are very close. Because the gravitational force makes the space of the interacting matter as small as possible, that is, the greater the density of the material on the earth's surface under the earth's gravity, the greater the gravitational force, the more it tends to the center of the earth, then those substances with low density will be far away from the center of the earth, like the less dense air will overflow from the liquid surface, or the relative density of the ball and the ship that occupies a large space is also very small, it will be repelled by the electron repulsion of the outer layer of the liquid surface molecule, and hold up on the surface of the liquid, or float in the air. This is the combined effect of the electromagnetic force and gravity between the molecules, where the gas and liquid exert pressure and buoyancy on the object placed into it.
To sum up, buoyancy is the result of the balance of suction and repulsion, and it is produced by the combined action of electromagnetic force and gravity among the four fundamental forces. On the Earth's surface, due to the gravitational pull of the Earth, the denser matter will move towards the center of the Earth, while the less dense material will move away from the Earth's core. Substances with less density, such as air and water, are lifted up on the surface of the liquid or float in the air under the combined action of intermolecular electromagnetic force and gravity, which is the cause of buoyancy. Similarly, an object placed in a liquid or gas is also subject to buoyancy and is equal in magnitude to the weight of the liquid or gas dislodged by the object, in the opposite direction. This is also the reason why objects are capable of suspending or floating in liquids or gases. In short, the understanding of buoyancy needs to be considered from multiple perspectives, including the density of matter, the interaction forces between molecules, the role of fundamental forces, and so on. Only in this way can we truly grasp the essence and law of buoyancy and better apply it to actual production and life.