Mass is the property of an object that is a measure of its inertia,
the amount of matter it contains, and its influence in a gravitational field. The mass of an
object never changes. Mass is resistant to change in a state of motion. Most scientists believe
that mass stands for a measurement of inertia, or resistance to change in the state of motion,
a property of all matter.
Inertia is the tendency of a stationary object to stay at rest, and
of a moving object to continue moving at a constant speed and in the same direction. The
inertial mass of an object is a measure of the object's resistance to motion by some external
force. One object has twice as much inertial mass as another object if it offers twice as much
force opposition to the same motion or acceleration.
The greater the mass of an object, the more
difficult it is to change its state of rest or motion. For example a jet has a greater mass than
a car. Therefore, it takes more force to stop a jet than to stop a car if both are moving at
the same speed.
The mass of an object may be determined by comparing the object on an equal arm scale or pan
balance scale with a set of standard masses. In this way, the gravitational factor is eliminated.
Because the numerical value for the mass of an object is the same anywhere in the universe, it
is used as a basis of reference for many physical measurements such as density or heat capacity.