*Reference:** Beginning Physics I*

**CHAPTER 13****: FLUIDS AT REST (HYDROSTATICS)**

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**KEY WORD LIST**

**Hydrostatics, Density, Specific Gravity, Pressure, Hydrostatic Pressure, Gauge Pressure, Hydraulic Press, Open-Tube Manometer, Barometer, Archimedes’ Principle,** **Surface Tension, Capillarity**

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## GLOSSARY

For details on the following concepts, please consult **CHAPTER 13****.**

**HYDROSTATICS**

Origin: “still water.” Hydrostatic is that branch of physics that deals with the static fluids, usually confined to the equilibrium and pressure of liquids.

**DENSITY**

The density *d* of any substance is defined as the mass per unit volume of the substance. If we have a uniform sample of materials (solid, liquid, or gas) of mass *M* and volume V, then

**SPECIFIC GRAVITY**

The *specific gravity* of a substance is defined as the ratio of the density of the substance to that of water.

**PRESSURE**

The *pressure P* on any surface is defined as the force per unit area acting perpendicular to that surface

**HYDROSTATIC PRESSURE**

The *hydrostatic pressure* is the pressure in a fluid at rest.

- For any point in a fluid at rest, the pressure on one side of a small surface is the same as the pressure on other side.
- The pressure at a given point in the fluid at rest has a definite value that represents the force per unit area on a small surface placed at that point, oriented in any arbitrary direction.
- The pressure in a fluid at rest is the same at all points on a horizontal plane.
- The pressure in a fluid at rest varies only with the depth in accord with the equation,

**GAUGE PRESSURE**

The *gauge pressure* is the difference between the actual, or absolute pressure *P* in a fluid, and the pressure exerted by the atmosphere *P _{A}*, which pervades the surface of the earth.

**HYDRAULIC PRESS**

**OPEN-TUBE MANOMETER**

**BAROMETER**

**ARCHIMEDES’ PRINCIPLE**

The fact that the buoyant force equals the weight of the displaced liquid is called the *Archimedes’ Principle: *the law of buoyancy.

**SURFACE TENSION**

Surface tension is caused by the pull of the molecules below the surface of the liquid on the molecules at the surface. This tends to pull the surface into a smooth and compact layer. The surface tension is defined as force per unit length exerted by a liquid surface on an object, along its boundary of contact with the object. This force is parallel to the liquid surface and perpendicular to the boundary line of contact. For a straight boundary of length *L* and a total force *F* we have

**CAPILLARITY**

Because of adhesion, the water surface gets pulled toward the wall of the container and bends upward at the point of contact.

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