Specific gravity, in fluid mechanics, is the ratio of a substance's density to the density of a reference substance, typically water for liquids and air for gases. It's a dimensionless quantity, meaning it has no units.
Essentially, specific gravity tells you how much denser or less dense a substance is compared to the reference substance. This is crucial in various engineering applications.
Here's a more detailed breakdown:
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Definition: Specific Gravity (SG) is defined as:
SG = (Density of substance) / (Density of reference substance)
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Reference Substance:
- For liquids and solids: Water at a specified temperature (usually 4°C or 60°F)
- For gases: Air at a specified temperature and pressure
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Formula: The formula for specific gravity is typically expressed as:
SG = ρsubstance / ρreference
Where:
- ρsubstance is the density of the substance being measured.
- ρreference is the density of the reference substance.
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Why is it useful?
- Density Comparison: Provides a direct comparison of the density of different substances relative to a common reference.
- Unitless: Because it's a ratio, specific gravity is dimensionless, making it easy to compare values across different unit systems.
- Applications: Used in various fields, including:
- Fluid Mechanics: Calculating buoyancy and hydrostatic pressure.
- Chemistry: Identifying and characterizing substances.
- Engineering: Designing pipelines, tanks, and other fluid-handling equipment.
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Example: If a liquid has a specific gravity of 1.5, it means it is 1.5 times denser than water.
| Property | Description |
|---|---|
| Definition | Ratio of a substance's density to the density of a reference substance. |
| Dimension | Dimensionless (no units) |
| Reference Liquid | Water (usually at 4°C or 60°F) |
| Reference Gas | Air (at a specified temperature and pressure) |
| Formula | SG = ρsubstance / ρreference |
| Usefulness | Allows for easy comparison of densities, crucial for calculations related to buoyancy, hydrostatic pressure, and substance identification in various engineering and scientific applications. |
| Importance | Indicates whether a substance will float or sink in water. Substances with SG < 1 will float; SG > 1 will sink. |
In summary, specific gravity is a simple yet powerful tool in fluid mechanics and related fields for quickly assessing and comparing the densities of different substances.
