# Matter (chemistry)/Addendum

## Mass

*See*: Mass

Mass gives a measure of the quantity of matter in an object, expressed in kilograms (kg), a basic unit of the International System of Units (SI units). Three related measures of mass exist, referred to as 'inertial mass', 'passive gravitational mass', and 'active gravitational mass'. Physicists have established that the three measures give equivalent values despite their different conceptual bases.

Inertial mass relates to a quantity of matter's resistance to motion in response to an applied force, resistance measured in terms of the degree of acceleration it undergoes in response to the applied force. For a given force, an object with a larger mass accelerates more slowly than an object with a smaller mass. For an iron block to achieve the same acceleration of a wood block requires a larger force than that acting on the wood block. Newton´s Second Law of Motion formulates the mass: force equals mass times acceleration, F=ma, mass expressed in kilograms, force expressed in newtons, and acceleration expressed in meters per second per second. From the chemist´s Newtonian perspective, one cannot create mass or destroy it, consequent to the law of conservation of mass.^{[1]}

Passive gravitational mass gives a measure of the quantity of matter in virtue of its reference to the property of an object to react to a gravitational field, that is, to react by attraction to another mass that is generating a mass-attracting force, a reaction which Newton called gravitation. The magnitude of the force attracting the object measures its weight, which increases with larger attracting masses, but the object´s mass remains constant, indicating no fixed weight for any given quantity of matter in an object.

Active gravitational mass gives a measure of the quantity of matter in virtue of its reference to the property of an object to create a field of force surrounding it that attracts another object — its property of creating a so-called gravitational field.

The equivalence of inertial mass and passive gravitational mass derives from Newton´s law of universal gravitation and the observation that different masses accelerate equally when let loose from the same height in a given gravitational field. The equivalence of passive and active gravitational mass derives both from Newton´s law of universal gravitation, Newton´s law of action and reaction,^{[2]} and the observation that one cannot shield an object from the force of gravity. The derivations are the provenance of physics.^{[3]}

### References

- ↑
**Note:**If one takes Einstein´s theory of special relativity into consideration, as a more accurate description of reality, mass increases as its velocity increases, hardly detectable as a rocket reaches Earth escape velocity, but hugely as the rocket approaches the speed of light. The theory of special relativity also predicts that mass need not obey the law of conservation of mass, because mass and energy exhibit two manifestations of the same thing, potentially enabling conversion of mass to energy, as in the nuclear reactions involved in generation of atomic energy, or energy to mass, as in the generation of hydrogen atoms from the energy released by the Big Bang that originated our universe. - ↑ The law of action and reaction states that two interacting objects apply equal forces to one another, equal in magnitude and opposite in direction — as in two colliding billiard balls.
- ↑ Dunsby P. Mass in Newtonian Theory. Online course on relativity: Chapter 5.
- An especially lucid, if somewhat technical, demonstration of the equivalances of the three concepts of mass.