# Mass and Its Measurement

Mass is
defined as the amount of matter in an object or as the measure of resistance to
acceleration of an object's (a change in its state of motion) when a net force
is applied.

It also
determines the strength of its mutual gravitational attraction to other bodies.

The basic 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, rather than comparing it directly
with known masses.

An object on
the Moon would weigh less than it does 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) determines the strength of this
force.

However, at very high speeds, special
relativity postulates 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 that some of these phenomena could be independent of
each other,current experiments have found no difference in results, whatever
way is 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 exerted on an object in a
known gravitational field.

The mass of
an object determines its acceleration in the presence of an applied force. The
inertia and the inertial mass describe the same properties of physical bodies
at the qualitative and quantitative level respectively, by other words, the
mass quantitatively describes the 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 is subject to an attractive force Fg =
GmAmB/r2, where G = 6.67×10−11 N kg−2 m2 is the "universal gravitational
constant". This is sometimes referred to as gravitational mass.[note 1]
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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