## What is Mass and Weight

The** mass** of an object is a fundamental property of the object. It is a numerical measure of its **inertia **and the measure of an object’s resistance to acceleration when a force is applied. It is also a fundamental measure of the amount of matter in the object. The greater the mass, the greater the force needed to cause a given acceleration. This is reflected in **Newton’s second law **(F=ma).

The **mass** of a certain body will remain constant even if the gravitational acceleration acting upon that body changes. For example, on earth an object has a** certain mass** and a **certain weight**. When the same object is placed in outer space, away from the earth’s gravitational field, its mass remains the same, but it is now in a “weightless” condition. This means in this condition it will weight zero, because gravitational acceleration and, thus, force will equal to zero.

**Mass and weight are related**: Bodies having large mass also have large weight. A large stone is hard to throw because of its large mass, and hard to lift off the ground because of its large weight. To understand the relationship between mass and weight, consider a freely falling stone, that has an acceleration of magnitude g (g = 9.81 m/s^{2} is the acceleration due to Earth’s gravitational field). Newton’s second law tells us that a force must act to produce this acceleration. If a 1 kilogram stone falls with an acceleration of the required force has magnitude:

*F = ma = 1 [kg] x 9.81 [m/s*^{2}*] = 9.8 [kg m/s*^{2}*] = 9.8 N*

## What is Density

**Density** is defined as the **mass per unit volume**. It is an **intensive property**, which is mathematically defined as mass divided by volume:

**ρ = m/V**

In words, the density (ρ) of a substance is the total mass (m) of that substance divided by the total volume (V) occupied by that substance. The standard SI unit is **kilograms per cubic meter** (**kg/m ^{3}**). The Standard English unit is

**pounds mass per cubic foot**(

**lbm/ft**). The density (ρ) of a substance is the reciprocal of its

^{3}**specific volume**(ν).

**ρ = m/V = 1/ρ**

**Specific volume** is an** intensive variable**, whereas volume is an extensive variable. The standard unit for specific volume in the SI system is cubic meters per kilogram (m^{3}/kg). The standard unit in the English system is cubic feet per pound mass (ft^{3}/lbm).

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