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One common method of defining mass is to
say that it is the quantity of matter an object possesses. For example,
a small rock has a fixed, unchanging quantity of matter. If you were to
take that rock to the Moon, to Mars, or to any other part of the
universe, it would have the same quantity of matter—the same mass—as
it has on Earth.
Mass is sometimes confused with weight.
Weight is defined as the gravitational attraction on an object by some
body, such as Earth or the Moon. The rock described above would have a
greater weight on Earth than on the Moon because Earth exerts a greater
gravitational attraction on bodies than does the Moon.
Mass
and the second law
A more precise definition of mass can be
obtained from Newton's second law of motion. According to that law—and
assuming that the object in question is free to move horizontally
without friction—if a constant force is applied to an object, that
object will gain speed. For example, if you hit a ball with a hammer
(the constant force), the ball goes from a zero velocity (when it is at
rest) to some speed as it rolls across the ground. Mathematically, the
second law can be written as F = m · a, where F is the force used to
move an object, m is the mass of the object, and a is the acceleration,
or increase in speed of the object.
Newton's second law says that the amount of
speed gained by an object when struck by a force depends on the quantity
of matter in the object. Suppose that you strike a bowling ball and a
golf ball with the same force. The golf ball gains a great deal more
speed than does the bowling ball because it takes a greater force to get
the bowling ball moving than it does to get the golf ball moving.
This fact provides another way of defining
mass. Mass is the increase in speed of an object provided by some given
force. Or, one can solve the equation above for m, the mass of an
object, to get m = F ÷ a. A kilogram, for example, can be defined as
the mass that increases its speed at the rate of one meter per second
when it is struck by a force of one newton.
Units of mass
In the SI system of measurement (the
International System of Units), the fundamental unit of mass is the
kilogram. A smaller unit, the gram, is also used widely in many
measurements. In the English system, the unit of mass is the slug. A
slug is equal to 14.6 kilograms.
Scientists
and nonscientists alike commonly convert measurements between kilogram
and pounds, not kilograms and slugs. Technically, though, a
kilogram/pound conversion is not correct since kilogram is a measure of
mass and pound a measure of weight. However, such measurements and such
conversions almost always involve observations made on Earth's surface
where there is a constant ratio between mass and weight.
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