Mechanics: Mass

Definition of Mass

Mass is a fundamental property of matter that quantifies the amount of matter in an object.
It is often confused with weight, but these two are different. Weight is a force caused by gravity acting on mass, whereas mass is a property inherent to the object itself.
Mass is an intrinsic property that does not change irrespective of location, be it on Earth, Moon or in outer space.
The standard unit of mass in the International System of Units (SI) is the kilogram (kg).

Mass governs the behaviours of objects in classical mechanics through properties like inertia and momentum. It strongly influences gravitational attraction between bodies as well.
It plays a fundamental role in the second law of motion, which states force = mass × acceleration (F = ma). In this context, mass acts as a resistance to change in velocity.
The gravitational potential energy an object possesses due to its position in a gravitational field is given by mgh, where m is mass, g is acceleration due to gravity and h is the height above the reference point.

Mass is usually measured using a balance, not a scale. While scales measure weight, the force exerted by mass in the presence of gravity, balances compare the mass of two or more objects.

Mass and energy are two fundamental, interconnected aspects of the universe. Einstein’s theory of relativity proposed in the famous equation E=mc² establishes this equivalence. Here, E is energy, m is mass, and c is the speed of light.
According to this equation, mass can be converted to energy and vice versa. This underpins the principle behind nuclear reactions where small amounts of mass are transformed into a large amount of energy.

In quantum physics, particles are often assigned a rest mass, which is the mass of the particle when it is not moving relative to the observer.
Different types of particles have different rest mass. For example, a photon, the particle of light, has zero rest mass, but particles like electrons, protons, and neutrons all have a measurable rest mass.
The concept of mass becomes more complex in quantum physics, which incorporates wave-particle duality; particles can demonstrate both a wave-like and a particle-like nature. The mass (and its equivalent energy) then relates to the frequency of the wave.