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# What is the definition of mass?

Mass is a measurement of how much matter there is in a body.

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In classical physics, mass is generally described as either

inertial mass or gravitational mass, which for all common purposes,

are the same. Inertial mass (which leads to momentum) is a

measurement of a body's resistance to a change in velocity when an

external force is applied to that body. Gravitational mass (which

leads to weight) is the magnitude of a body's interaction with a

known gravitational field.

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In relative physics, mass has two components: the classical

mass, described above, also known as rest mass (or invariant mass),

and relativistic mass. A body's rest mass remains constant, and its

relativistic mass is zero when the the body is not moving (i.e., at

rest). A body's relativistic mass increases as the body's velocity

approaches the speed of light. Relativistic mass does not become

significant until the body's velocity exceeds 1/10th the speed of

light (0.1c). At 0.1c, the body's mass is 1.005 times its rest

mass, meaning the relativistic mass is 0.5% of the rest mass. At

0.86c, relativistic mass equals rest mass, meaning the body's mass

is twice its rest mass. It should be noted that the body's

effective inertial mass is twice the rest mass at 0.86c - meaning

it has twice the momentum it would have if the body had rest mass

alone. In other words, the body in motion at 0.86c is twice as hard

to speed up, slow down, or change direction than if it had had only

rest mass. The relativistic mass increases asymptocically to

infinity as the body's velocity approaches the speed of light.

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In quantum physics, mass can be described as the amount of

potential energy contained within a body, and is a measurement of

how much energy would be liberated should that mass be converted

into energy through mass annihilation using the famous E=mc2

equation.

It could also mean church.

the density or volume of matter

The word mass has many different meanings.

Mass (noun) - a large body of matter

Mass (adjective) - affecting a large number of people

Mass (verb) - to assemble as one group or large body

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