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In physics, mass is a property of a physical body. It is generally a measure of an object's resistance to changing its state of motion when a force is applied. It is determined by the strength of its mutual gravitational attraction to other bodies, its resistance to acceleration or directional changes, and in the theory of relativity gives the mass–energy content of a system. The SI unit of mass is the kilogram (kg).
Mass is not the same as weight, even though we often calculate an object's mass by measuring its weight with a spring scale instead of comparing it to known masses. An object on the Moon would weigh less than it would on Earth because of the lower gravity, but it would still have the same mass. This is because weight is a force, while mass is the property that (along with gravity) causes this force.
In Newtonian physics, mass can be generalized as the amount of matter in an object. However, at very high speeds or for subatomic particles, special relativity shows that energy is an additional source of mass. Thus, any stationary body having mass has an equivalent amount of energy, and all forms of energy resist acceleration by a force and have gravitational attraction. In addition, "matter" is a loosely defined term in science, and thus cannot be precisely measured.
There are several distinct phenomena which can be used to measure mass. Although some theorists have speculated some of these phenomena could be independent of each other, current experiments have found no difference among any of the ways used to measure mass:
Inertial mass measures an object's resistance to being accelerated by a force (represented by the relationship F = ma).
Active gravitational mass measures the gravitational force exerted by an object.
Passive gravitational mass measures the gravitational force experienced by an object in a known gravitational field.
Mass–energy measures the total amount of energy contained within a body, using E = mc2.
The mass of an object determines its acceleration in the presence of an applied force. This phenomenon is called inertia. According to Newton's second law of motion, if a body of fixed mass m is subjected to a single force F, its acceleration a is given by F/m. A body's mass also determines the degree to which it generates or is affected by a gravitational field. If a first body of mass mA is placed at a distance r (center of mass to center of mass) from a second body of mass mB, each body experiences an attractive force Fg = GmAmB/r2, where G = 6989667000000000000♠6.67×10−11 N kg−2 m2 is the "universal gravitational constant". This is sometimes referred to as gravitational mass. Repeated experiments since the 17th century have demonstrated that inertial and gravitational mass are identical; since 1915, this observation has been entailed a priori in the equivalence principle of general relativity.

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