Mass is the amount of matter in a body. An object has the same mass where ever it is.
The SI unit of mass is the kilogram, written as kg. There are also many other units of mass, such as: grams (g), tonnes (t), pounds (lb.), and ounces (oz.). Other units of mass used in science and engineering include slugs, atomic mass units, Planck masses, Solar masses, and eV/c2. The last unit is based on the electron volt (eV), which is usually used as a unit of energy.
Various types of scales, including balances, are used to measure mass.
What gives mass.
Physicists do not know what gives an object mass. The Standard Model of particle physics thinks that mass is caused by the Higgs field. Physicists are trying to find the Higgs field now using the Large Hadron Collider at CERN.
Mass can change.
In physics, Special Relativity shows that the mass of an object becomes bigger when the object moves very fast. As the speed gets close to the speed of light the mass becomes very big.
Some things that do not have mass on their own have mass because of their movement. This is true for light - a light photon has no mass, but its energy can act as mass when it hits something.
In chemistry mass does not change. The mass of chemicals before a chemical reaction always equals the mass of chemicals after. This is called the Law of Conservation of Mass.

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