Weight is a measure of the centripetal force, or thrust exerted on,

and/or by objects; bodies, or masses of the particles [atoms and

molecules] that comprise the mass of the substances that we call

matter; when they are at rest on the terra firma surface of Earth, or

on a similar planet.

This force is primarily due to gravitation; where all matter is

continuously gravitating toward a common center of mass; at a rate

that varies inversely as the distance separating these centers of

mass; as well as being affected by the centrifugal effect due to the

planet's rotation.

That is at Earth's equator where the rotation is greatest, the

centrifugal effect is greatest, and therefore causes the centripetal

weight there to be least. Weight will also vary with elevation, and is

less on hills than in valleys, because they are further from Earth's

center than sea level.

Units of weight are units of force: In the International System of

Units (SI), the "modern" metric system, the units of force are the

dyne and the newton. In the foot-pound-second system used in the

United States, units of force customarily include the ounce and the

pound. One pound being equal to 4.448 newtons; which is the weight of

0.454 kilogram.

Newton found the "mass" of an object to be equal to it's "bulk and

density, conjointly", as well as being the ratio of its weight [w],

divided by the acceleration at which it will free fall [g] at the

location where it is weighed.

He related this "gravitational mass" [w/g] to mass [m] in general as

being equal to "inertial mass" [f/a]; which is the ratio of the net

force [f], divided by the acceleration [a] that it causes _anywhere_,

_anytime_: Where through algebra: f = wa/g, and w = fg/a: Where the

mass is incidental; since it's just two different ways of saying the

same thing: That mass is a ratio of force to acceleration (and/or

deceleration), and is a measure of inertia.

For commercial and everyday purposes, weight is commonly used to mean

the quantity of matter in an object. When people use weight in this

sense, they measure it on weight scales. The kilogram is the SI's base

unit of mass; where one pound is the weight of 0.454 kilogram.

[This is a rewrite of the World Book's article on weight., and in my

humble opinion is considerably truer.]

A slug at the earth's surface has a weight of about 32.174#. Above the

earth's surface it weighs less and less. Contrary to popular belief ,

below the earth's surface (in a mine or borehole) it has been found

that a mass of matter weighs more than at the surface. The reason is

that weight increases to a maximum as it approaches a common center of

mass!

Earth does not _attract matter_ from Earth's center; making it

weightless there: Instead the matter around Earth's center gets

heavier, the closer it is to that center.

In orbit around the earth astronauts feel no weight at all, even

though they are still as massive as they were on the ground. This is

not due to the distance they are from the earth, which may be only a

hundred miles or so. They are weightless because they are in free

fall, as you would be if you fell off a cliff. They don't fall

straight down, however, because they are traveling forward at about 30

times the speed of sound (mach 30), and keep falling around the earth!

Things are less heavy on the moon because they fall about six times

slower there: Mass and/or gravitational inertia are constant because

they are a _ratio_ of their weight, divided by the rate at which they

will free fall; wherever they are. The denominator of the ratio [w/g]

is the acceleration of free fall on the moon, and is only about one

sixth of its value here on Earth: That's what causes things to weigh

only one sixth as much as they do on Earth.

------------------------------------------------------------------------

If the people who determined that grams and kilograms were constants

had stopped to think, they'd have realized this; that it's a

scientific fact that weight varies at various locations; in proportion

to the acceleration of free fall [g], which also varies at various

locations: But they didn't; they were in too much of a rush to outdo

the British, and they're still trying. If only they could get the free

people of the U.S. to go along with the scam; but it won't happen:

Ever!

A few further comments should be added about the force called weight

which is a source of much confusion to many students, and teachers of

physics. Weight is the mutual force exerted between bodies on the

ground and the resisting force exerted by the ground; which restrain

each other from gravitating further toward their common center of

mass; which for all intents and practical purposes is the center of

the planet:

The force of gravity acting upon an object is sometimes referred to as

the mass of the object. Many students and teachers of physics confuse

weight with mass. The mass of an object refers to the amount of matter

that is contained by the object; the weight of an object is the force

of gravity acting upon that object. Mass is related to "how much stuff

is there" and weight is related to the pull of the Earth (or any other

planet) upon that stuff. The mass of an object (measured in kg) will

be the same no matter where in the universe that object is located.

Mass is never altered by location, the pull of gravity, speed or even

the existence of other forces. For example, a 2-kg object will have a

mass of 2 kg whether it is located on Earth, the moon, or Jupiter; its

mass will be 2 kg whether it is moving or not (at least for all

practical purposes); and its mass will be 2 kg whether it is being

pushed or not.

On the other hand, the weight of an object (measured in Newtons) will

vary according to where in the universe the object is. Weight depends

upon which planet is exerting the force and the distance the object is

from the planet. Weight, being equivalent to the force of gravity, is

dependent upon the value of g. On earth's surface g is 9.8 m/sec^2

(sometimes approximated as 10 m/sec^2). On the moon's surface, g is

1.7 m/sec^2. Go to another planet, and there will be another g value.

Furthermore, the g value is inversely proportional to the distance

from the center of the planet. So if we were to measure g at a

distance of 400 km above the earth's surface, then we would find the g

value to be less than 9.8 m/sec^2:

Always be cautious of the distinction between mass and weight. It is

the source of much confusion for many students and physicists.