Specific weight facts for kids

The specific weight (also known as the unit weight) tells us how much a certain amount of a material weighs for its size. Think of it as how heavy something is for the space it takes up. The special symbol for specific weight is γ (which is the Greek letter gamma).

A common example is the specific weight of water on Earth. At 4° Celsius, water's specific weight is about 9.807 kilonewton per cubic meter. This means a cubic meter of water (a cube 1 meter long, 1 meter wide, and 1 meter high) weighs about 9.807 kilonewtons.

What is Specific Weight?

Specific weight helps us understand how heavy different materials are compared to their volume. It's different from just weight because it considers the space the material fills. For example, a big balloon might weigh very little, but a small rock can be quite heavy. Specific weight helps us compare them fairly.

How is Specific Weight Measured?

To find the specific weight of something, you need two main things:

• Weight: How heavy the material is.
• Volume: How much space the material takes up.

You then divide the weight by the volume. The units for specific weight usually involve a unit of force (like Newtons or pounds) divided by a unit of volume (like cubic meters or cubic feet).

Why is it Important to Know Specific Weight?

Understanding specific weight is super useful in many areas, especially in science and engineering.

• Building and Construction: Engineers use specific weight to figure out how much different building materials will weigh. This helps them design strong buildings, bridges, and other structures that won't collapse.
• Ship Design: When designing ships, knowing the specific weight of water is crucial. It helps determine how much cargo a ship can carry without sinking.
• Fluid Mechanics: In the study of liquids and gases, specific weight helps scientists understand how fluids behave, like how they flow or how much pressure they create.
• Soil Mechanics: Geologists and engineers use specific weight to understand different types of soil. This is important for building foundations and roads.

Specific Weight vs. Density: What's the Difference?

You might have heard of density, which is very similar to specific weight but not quite the same.

• Density is the mass per unit volume. Mass is how much "stuff" is in an object.
• Specific weight is the weight per unit volume. Weight is the force of gravity pulling on that "stuff."

The main difference is that weight depends on gravity, while mass does not. So, the specific weight of an object would be different on the Moon than on Earth because the Moon has less gravity. However, its density would stay the same.

How are They Related?

Specific weight and density are connected by gravity. You can find the specific weight by multiplying the density of a material by the acceleration due to gravity (which is about 9.8 meters per second squared on Earth).

Examples of Specific Weight

Different materials have different specific weights.

• Water: As mentioned, water has a specific weight of about 9.807 kilonewtons per cubic meter on Earth.
• Air: Air is much lighter than water. Its specific weight is much smaller, around 0.012 kilonewtons per cubic meter at sea level.
• Steel: Steel is very heavy. Its specific weight can be around 77 kilonewtons per cubic meter, depending on the type of steel.

These differences are why a steel ship floats (because of its design and the water it displaces), but a solid block of steel sinks!

See also

In Spanish: Peso específico para niños