Mass

The mass of an object is a measure of the amount of matter it has.^[1] A mountain has a greater mass than a rock, while mass is different weight, which is the vector (directional) product of mass and gravitational acceleration, e.g. 9.81 ms^-2. Weight is sometimes called the force of gravity.

The mass of an object is measurable when a force is exerted on the object. If the mass is greater, the object will have less acceleration (changes in velocity), which is sometimes called inertial mass as it measures the inertia.^[2]

A gigantic mass, like our planet Earth, attracts such a smaller mass as human being to keep the human being from floating away from the Earth's surface. "Mass attraction" is another word for gravity, a force existing between all matters. When measuring the force of gravity exerted on an object, its gravitational mass can be found. Tests of inertial and gravitational mass show that they are the same or almost the same.^[2]

The unit of mass in the International System of Units is the kilogram, which is represented by the symbol 'kg'. Fractions and multiples of this basic unit include the gram (one thousandth of a kg, symbol 'g') and the tonne (one thousand kg), amongst many others.

In some fields or applications, it is convenient to use different units to simplify the discussions or writings. For instance,

• Atomic physicists deal with the tiny masses of individual atoms and measure them in atomic mass units.
• Jewelers normally work with small jewels and precious stones where masses are traditionally measured in carats, which correspond to 200 mg or 0.2 g.
• The masses of stars are very large and are sometimes expressed in units of solar masses.

Traditional units are still in encountered in some countries: imperial units such as the ounce or the pound were in widespread use within the British Empire. Some of them are still popular in the United States, which also uses units like the short ton (2,000 pounds, 907 kg) and the long ton (2,240 pounds, 1010 kg), not to be confused with the metric ton (1,000 kg).

Mass is an intrinsic property of the object: it does not depend on its volume, or position in space, for instance. For a long time (at least since the works of Antoine Lavoisier in the second half of the eighteen century), it has been known that the sum of the masses of objects that interact or of the chemicals that react remain conserved throughout these processes. This remains an excellent approximation for everyday life and even most laboratory work.

However, Einstein has shown through his special theory of relativity that the mass m of an object moving at speed v with respect to an observer must be higher than the mass of the same object observed at rest m₀ with respect to the observer. The applicable formula is

${\displaystyle m={\frac {m_{0}}{\sqrt {1-(v^{2}/c^{2})}}}}$

where c stands for the speed of light. This change in mass is only important when the speed of the object with respect to the observer becomes a large fraction of c.

For further reference,see the Gluon field and Higgs boson

In atomic nuclei, example in protons and neutrons,the residual mass comes from the binding kinetic and potential energy of the quarks and gluon field. An analogy to go along with is to think of the 3 quarks as balls with the gluons as a spring connecting the quarks. This mass accounts for 99% of the mass of these baryons with the remaining 1% coming from the individual quarks which comes from quantum interactions with the Higgs field.

1. ↑ Tsokos, K. A. (2005). Physics for the IB Diploma. Cambridge, United Kingdom: Cambridge University Press. p. 63. ISBN 9780521604055.
2. ↑ ^{2.0 2.1} Knight, Randall Dewey (2003). Physics for scientists and engineers with modern physics : a strategic approach. San Francisco: Pearson/Addison-Wesley. p. 349. ISBN 0-321-24329-3. OCLC 54427199.

• Mass versus weight
• Center of mass
• Gravity
• Density
• Body mass index
• Momentum
• Advanced topics:
  • Standard Model
  • Higgs field.