For: Orders of magnitude (mass)

Mass is a concept used in the physical sciences to explain a number of observable behaviors. In everyday usage, it is common to identify mass with the observable behaviors that result from mass. In particular, mass is commonly identified with weight. But, in physics and engineering, weight means the strength of the gravitational pull on the object; that is, how heavy it is, measured in units of newtons. In normal situations, the weight of an object is proportional to its mass, which usually makes it unproblematic to use the same word for both concepts. However, the distinction between mass and weight becomes important for measurements with a precision better than a few percent (due to slight differences in the strength of the Earth's gravitational field at different places), and for places far from the surface of the Earth, such as in space or on other planets.

The concept of mass was introduced in, and is central to, Isaac Newton's explanation of gravitation and inertia. Prior to Newton's time, the orbits of the planets, the gravitational acceleration of objects near the surface of the earth, and the inertial resistance of objects to changing their motion were viewed as distinct and potentially unrelated phenomena. However, Isaac Newton united these phenomena using a single underlying concept called mass. Since Newton's time, the concept of mass has grown to include explanations for both quantum and relativistic effects.

Units of mass

The primary instrument used to measure mass is the scale or balance scale. In the SI system of units, mass is measured in kilograms, kg. Many other units of mass are also employed, such as:

• gram: 1 g = 0.001 kg (1000 g = 1 kg)
• tonne: 1 tonne = 1000 kg
• MeV/c² (Typically used to specify the mass of subatomic particles. See Mass-energy equivalence)

Outside the SI system, a variety of different mass units are used, depending on context, such as the:

• Slug
• Pound
• atomic mass unit
• Planck mass
• solar mass

Because of the relativistic connection between mass and energy (see mass in special relativity), it is possible to use any unit of energy as a unit of mass instead. For example, the eV energy unit is normally used as a unit of mass (roughly ) in particle physics. A mass can sometimes also be expressed in terms of length. Here one identifies the mass of a particle with its inverse Compton wavelength ( ).

Summary of concepts of mass

In physical science, one may distinguish conceptually between at least seven types of mass, or seven physical phenomena that can be explained using the concept of mass:

• Inertial mass is a measure of an object's resistance to changing its state of motion when a force is applied. It is determined by applying a force to an object and measuring the acceleration that results from that force. An object with small inertial mass will accelerate more than an object with large inertial mass when acted upon by the same force. One says the body of greater mass has greater inertia.