Define specific weight.

Short Answer:

Specific weight is the weight per unit volume of a substance. It shows how much weight is contained in a certain volume. It is generally denoted by the Greek letter “γ” (gamma). The specific weight depends on both the density of the substance and the gravitational acceleration. It is expressed in units of newton per cubic meter (N/m³).

In simple words, specific weight tells us how heavy a material is for a given space. For example, the specific weight of water is about 9,810 N/m³ at standard gravity. It helps engineers to calculate the load, pressure, and buoyant forces in mechanical and civil applications.

Detailed Explanation :

Specific Weight

Specific weight is an important physical property that connects the weight of a substance to its volume. It tells how much weight is present in a particular volume of the material. The specific weight is different from density because density relates to mass per unit volume, while specific weight relates to weight per unit volume.

Mathematically, specific weight (γ) is given by the formula:

where,
γ = specific weight (N/m³)
W = weight of the substance (N)
V = volume of the substance (m³)

Since the weight (W) is equal to the product of mass (m) and gravitational acceleration (g), the formula can also be written as:

where,
ρ = density of the substance (kg/m³)
g = acceleration due to gravity (9.81 m/s² on Earth)

This equation shows that specific weight depends not only on the material’s density but also on the value of gravitational acceleration. Hence, specific weight changes with the location — for example, it will be different on Earth, Moon, or Mars because the value of gravity is different on each planet.

Unit and Dimensions

The SI unit of specific weight is newton per cubic meter (N/m³) because it represents the force (weight) acting on a unit volume.
In terms of dimensions,

This means it has the dimension of force per volume.

Examples of Specific Weight

  1. Water: The specific weight of water at 4°C is approximately 9,810 N/m³.
  2. Air: The specific weight of air at standard conditions is around 12 N/m³.
  3. Steel: The specific weight of steel is approximately 77,000 N/m³.
  4. Concrete: The specific weight of concrete is about 24,000 N/m³.

These examples show that heavier materials have a higher specific weight, while lighter materials have a lower specific weight.

Importance of Specific Weight in Engineering

  1. Design of Structures:
    Engineers use specific weight to calculate loads on buildings, bridges, and machines. It helps in determining how much weight a structure must bear safely.
  2. Fluid Mechanics:
    In fluids, specific weight helps determine the pressure and buoyant force. The pressure at a depth in a liquid depends directly on the specific weight of the fluid.
  3. Hydraulic Machines:
    In pumps, turbines, and dams, the specific weight of water or other fluids affects energy and flow calculations.
  4. Material Selection:
    When selecting materials for machines or vehicles, engineers consider specific weight to reduce weight and improve performance.

Difference Between Density and Specific Weight

  • Density (ρ): Mass per unit volume, measured in kg/m³.
  • Specific Weight (γ): Weight per unit volume, measured in N/m³.
    Both are related by the equation γ = ρ × g, meaning specific weight changes if gravity changes, but density remains constant.

Factors Affecting Specific Weight

  1. Temperature:
    Increasing temperature usually decreases density, and thus specific weight also decreases.
  2. Pressure:
    For gases, specific weight increases with pressure because the gas becomes denser.
  3. Gravitational Acceleration:
    On planets with lower gravity, specific weight will be less even if the material’s density is the same.
Conclusion

Specific weight is the measure of how heavy a substance is per unit volume. It depends on both the density of the material and gravitational acceleration. It plays a key role in engineering fields like fluid mechanics, structural design, and materials science. Understanding specific weight helps in accurate calculation of forces, pressures, and loads in real-world mechanical and civil systems.