What is a Stress Ball?

The volume that generates pressure on the surface of an object due to the weight of the liquid is called a pressure body. As shown in the figure, the pressure body is divided into a real pressure body (left) and a virtual pressure body (right). The pressure generated by the real pressure body acts vertically on the object surface; while the pressure generated by the virtual pressure body acts vertically on the object surface.

Pressure body

In hydraulics, the volume of a plumb plume between a curved surface and a free surface. The pressure body is surrounded by the following surfaces: the surface in question; the surface formed by the plumb lines passing through all points on the perimeter of the surface; the horizontal plane coinciding with the liquid surface. If the pressure body is actually filled with liquid, the direction of the vertical component force is downward.

The pressure body is a volume surrounded by three surfaces: the pressure surface itself, the vertical surface that is drawn to the free water surface around the pressure surface, and the free water surface or its extension surface. The pressure body on the same side as the pressure surface is called a real pressure body, and the water pressure is downward; the pressure body and the pressure surface are not on the same side as a virtual pressure body, and the water pressure is upward.

For example: A sphere is immersed in water, the center of the sphere is O, the upper hemisphere AEB (E is at the top of the ball), and the lower hemisphere AFB (F is at the bottom of the ball). According to the concept of pressure body, the pressure body of the upper hemispherical AEB is the water column between the upper hemisphere and the water surface, which is a solid pressure body, and the pressure is downward. It is a virtual pressure body, and the pressure is upward. After the cancellation of the two is a sphere, and it is a virtual pressure body. The water pressure is upward. This pressure body is multiplied by the density of water, and then multiplied by the acceleration of gravity, which is the buoyancy of the ball in water.

The general method for determining the pressure body: 1) First take the free liquid surface or its extension line;

2) Secondly take the surface itself;

3) Project the two ends of the curved surface to the free liquid surface (or its extension line) to obtain two projection lines.

4) The above four lines will enclose one or more closed volumes. The sum of the vectors of these volumes after considering the direction of force is the required pressure body.

For example

For example, for the complex two-dimensional surface on the right, using the above method, first take the liquid surface, then the surface itself, and finally draw two projection lines from the two ends of the surface to the liquid surface.

It can be seen that there are 3 enclosed volumes, and can be judged

Break the force direction of each enclosed volume. Then, a pressure body is obtained, as shown in the right figure.