Objects floating on the surface of fluid or immersed in fluid are subjected to the upward resultant force of hydrostatic pressure from all directions. Its size is equal to the gravity of the fluid displaced by the object. For example, if the gravity of a stone is greater than that of the same volume of water, it will sink to the bottom of the water. The gravity of wood or hull is equal to the weight of water displaced by the submerged part, so it floats on the water. The weight of the balloon is less than the gravity of the same volume of air, that is, the buoyancy is greater than gravity, so it will rise. This kind of force immersed in water or air and pulled up by water or air is called "buoyancy". For example, lifting a bucket of water from a well is lighter before leaving the water surface than after leaving the water surface, because the bucket is buoyed by the water. Not only water, but also all liquids, such as alcohol, kerosene or mercury, have buoyancy for objects immersed in it.

The cause of buoyancy can be analyzed by immersing a regular cube in liquid. The completely submerged object in this system is under the pressure of liquid from all directions, and it increases with the increase of depth. So the front, back, left, right, upper and lower surfaces of this regular cube are all under the pressure of liquid. Because the forces acting on the left and right sides correspond to each other, have the same area and the same depth in the liquid, the pressures on both sides are equal in magnitude and opposite in direction, and the two forces are balanced with each other. Similarly, the pressures acting on the front and rear sides are balanced with each other. However, due to the different depths of the upper surface and the lower surface in the liquid, the pressure is not equal. The pressure above is small, and the pressure below is large. The upward pressure below is greater than the downward pressure above. The pressure difference between a liquid and an object is the buoyancy of the liquid to the object. This force is equal to the weight of the liquid displaced by the object. When the top interface of the floating body does not touch the liquid, only the upward pressure acting on the bottom interface will produce buoyancy. As for the object located at the bottom of the container and in close contact with the bottom of the container, it can only be subjected to the liquid pressure acting downward on the surface of the object, so the object is not affected by buoyancy. This phenomenon is rare, because as long as there is a thin liquid film in the middle, the pressure can be transmitted, and there will be upward pressure on the bottom. If there is a pressure difference between the upper and lower surfaces of an object, the object will be floated.