Chapter 2 Forces in Liquids

Fluid Power - Intro to Hydraulics, Pneumatics, and how it all works
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CHAPTER 2 FORCES IN LIQUIDS The study of liquids is divided into two main parts: liquids at rest (hydrostatics) and liquids in motion (hydraulics). The effects of liquids at rest can often be expressed by simple formulas. The effects of liquids in motion are more difficult to express due to frictional and other factors whose actions cannot be expressed by simple mathematics. In chapter 1 we learned that liquids have a definite volume but take the shape of their containing vessel. There are two additional characteristics we must explore prior to pro- ceeding. Liquids are almost incompressible. For example, if a pressure of 100 pounds per square inch (psi) is applied to a given volume of water that is at atmospheric pressure, the volume will decrease by only 0.03 percent. It would take a force of approximately 32 tons to reduce its volume by 10 percent; however, when this force is removed, the water immediately returns to its original volume. Other liquids behave in about the same manner as water. Another characteristic of a liquid is the tendency to keep its free surface level. If the surface is not level, liquids will flow in the direction which will tend to make the surface level. LIQUIDS AT REST In studying fluids at rest, we are con- cerned with the transmission of force and the factors which affect the forces in liquids. Additionally, pressure in and on liquids and factors affecting pressure are of great im- portance. PRESSURE AND FORCE The terms force and pressure are used extensively in the study of fluid power. It is essential that we distinguish between the terms. Force means a total push or pull. It is the push or pull exerted against the total area of a particular surface and is expressed in pounds or grams. Pressure means the amount of push or pull (force) applied to each unit area of the surface and is expressed in pounds per square inch (lb/in²) or grams per square centimeter (gm/cm²). Pressure maybe exerted in one direction, in several directions, or in all directions. Computing Force, Pressure, and Area A formula is used in computing force, pressure, and area in fluid power systems. In this formula, P refers to pressure, F indicates force, and A represents area. Force equals pressure times area. Thus, the formula is written Equation 2-1. Pressure equals force divided by area. By rearranging the formula, this statement may be condensed into Equation 2-2. Since area equals force divided by pressure, the formula is written Equation 2-3. 2-1

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