Mass is a fundamental concept in physics that plays a crucial role in understanding the world around us. It is an inherent property of matter and is used to measure the amount of matter an object contains.

Mass has applications in various fields of physics, from classical mechanics to the theory of relativity and particle physics.

## Concept and definition of mass

Mass is defined as the measure of the amount of matter an object contains. It is a scalar property and its unit of measurement in the International System is the kilogram (kg).

The mass does not depend on the location of the object and is an intrinsic property of each particle or body. Furthermore, mass is additive, which means that the total mass of a system is the sum of the individual masses of its components.

### Units of mass

The standard unit of mass in the International System is the kilogram (kg). However, the Anglo-Saxon system also uses other units such as the pound (lb) or the ounce (oz).

One pound equals approximately 0.4536 kilograms and is commonly used in countries like the United States and the United Kingdom to measure the mass of objects and people.

One ounce equals approximately 28.35 grams. The ounce is mainly used to measure small amounts of mass, such as in the food industry.

## Importance of mass in classical physics

In the field of classical physics, mass plays a fundamental role. Here are some examples:

### Classical mechanics

In dynamics, mass plays a fundamental role in the equations of motion. According to Newton's second law, the acceleration of an object is directly proportional to the net force applied to it and inversely proportional to its mass.

This is expressed by the equation F = ma, where F is the force, m is the mass, and a is the resulting acceleration.

### Universal law of gravitation

According to Newton's universal law of gravitation, the gravitational force of attraction between two objects is directly proportional to the mass of both objects and inversely proportional to the square of the distance between them.

Therefore, the mass of an object determines its gravitational influence on other objects and its response to the gravitational force.

### Conservation of linear momentum

The conservation of linear momentum (also known as momentum) depends directly on mass.

The principle of conservation of linear momentum states that the sum total of the momenta of an isolated system is conserved in the absence of external forces. The mass of the objects involved in a system determines how momentum is distributed and conserved in the interactions between them.

### Collisions between objects

The mass of the colliding objects affects how momentum and energy are transferred during the collision.

The relationship between the masses of the objects and their velocities before and after the collision determines the outcome and the magnitude of the momentum and energy transfer.

### Oscillations and vibrations

The concept of mass is also present in the study of oscillations and vibrations.

For example, in a mass-spring system, the mass of the object attached to the spring determines the frequency and period of oscillation. The relationship between the mass and the stiffness of the spring influences the characteristics and behavior of the oscillations.

## Relationship between mass and energy

Within physics there are multiple concepts that closely relate mass and energy. In this section we highlight two examples:

### Einstein's energy equation

Alberty Einstein's equation, E = mc², states that an amount of energy (E) is equivalent to a mass (m) multiplied by the square of the speed of light (c²).

This revolutionary idea implies that mass can be converted into energy and vice versa, as seen in phenomena such as nuclear fission and nuclear fusion.

### Kinetic energy

Kinetic energy is defined as the energy associated with the movement of an object and depends on both its mass and its speed. The mathematical expression to calculate the kinetic energy (K) is K = (1/2)·m·v^{2} , where m is the mass of the object and v is its velocity.

The relationship between kinetic energy and mass is evident by looking at the formula. As the mass of an object increases, its kinetic energy also increases proportionally, as long as the speed remains constant.

Kinetic energy is an essential concept in mechanics and is used to describe a wide range of phenomena, from the motion of everyday objects to the study of subatomic particles in particle accelerators.