Mass explanation

MassA measure of the amount or quantity of matter. By international agreement the standard unit of mass, with which the masses of all other objects are compared, is a platinum-iridium cylinder of one kilogram. In countries that favor the English system (avoirdupois system- based upon a pound of 16 ounces) of measurement over the International System of Units (SI), the avoirdupois pound, a measure of mass, is used instead. Finally another unit of mass is the slug, which is associated with Imperial units and equals 14.59 kg (32.17 lbm).

In contrast to “weight”, mass remains constant regardless of its location, under ordinary circumstances. A satellite launched into space for example, weighs increasingly less the further it travels away from the earth. Its mass, however, stays the same. 

The mass of an object is determined by its volume and density.

Density of materials
Material Density (kg/m3)
Water 1,000 
Stainless Steel  7,982
Mild Steel 7,850
Titanium 4,520
Aluminium 2,705
Concrete 2,400
calc: Mass relationship with volume and density

Mathematically we can express this relationship as:

v (Volume) =
d (Density) =

So, if we wish to calculate the mass of a block of steel measuring 0.2m x 0.3m x 0.212m:
mass = 0.2m x 0.3m x 0.212m x 7,850 kg/m3 = 0.01272 m3 x 7,850 kgm-3 = 99.852kg ≈ 100kg

Notice that the dimensional units (m3 x m-3) cancel out to produce an answer in kilograms.

Mass definition in physics

Mass is a fundamental concept in physics that represents the amount of matter contained in an object. It is a scalar quantity, meaning it has magnitude but no specific direction. Mass is often denoted by the symbol “m” and is measured in units such as kilograms (kg) in the International System of Units (SI).

Key points about mass include:


Mass is directly related to an object’s inertia, which is its resistance to changes in motion. Objects with greater mass have greater inertia, meaning they are more resistant to changes in their state of rest or motion. This is described by Newton’s first law of motion, which states that an object at rest will remain at rest, and an object in motion will continue moving at a constant velocity, unless acted upon by an external force.

Conservation of Mass

Mass is conserved in isolated systems, according to the principle of conservation of mass. This principle states that the total mass of a closed system remains constant over time, even if physical or chemical changes occur within the system.

Different from Weight

Mass should not be confused with weight. While mass refers to the amount of matter in an object, weight is the force exerted on an object due to gravity. Weight is dependent on the gravitational field strength, whereas mass remains the same regardless of the location. Mass is measured in kilograms, while weight is typically measured in newtons (N).

Measurement of Mass

The mass of an object can be measured using various techniques and instruments. Common methods include using balances, scales, or mass measurement devices that compare the object’s mass to a known reference mass. The standard kilogram (kg) is defined by a specific mass artifact known as the International Prototype of the Kilogram, maintained by the International Bureau of Weights and Measures.

quiz: Weight is typically measured in
Energy Equivalence

According to Einstein’s famous equation E = mc², mass and energy are interrelated. Mass can be converted into energy and vice versa, as demonstrated by nuclear reactions and particle physics phenomena.

Inertia and Gravitational Force

Mass is also related to gravitational force. The gravitational force acting on an object is directly proportional to its mass. Objects with greater mass experience a stronger gravitational force when subjected to the same gravitational field.


Understanding the concept of mass is fundamental to various areas of physics, including mechanics, thermodynamics, and particle physics. It is a fundamental property of matter that influences the behavior and interactions of objects in the physical world.

video: Are Mass and Weight the same thing? | Physics | Don't Memorise
YouTube channel:Are Mass and Weight the same thing? | Physics | Don't Memorise
quiz: When mass and weight are equal?
For the object at rest
When divided by the acceleration of Gravity
quiz: What is the unit of mass according to International System of Units?
Platinum-iridium cylinder of 1 kilogram (kg)
Platinum-iridium cylinder of 1 pound (lb)
Platinum-iridium cylinder of 1 gram (g)
Platinum-iridium cylinder of 1 ounce (oz)
Sources nps technical rescue handbook 2014
Physics for roping technicians (2022)

For video sources please check the information under the video.
You are here in Content