In science and engineering, the weight of an object is the force on the object due to gravity. Its magnitude (a scalar quantity), often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g; thus: W = mg. When considered a vector, weight is often denoted by a bold letter W. The unit of measurement for weight is that of force, which in the International System of Units (SI) is the newton. For example, an object with a mass of one kilogram has a weight of about 9.8 newtons on the surface of the Earth, about one-sixth as much on the Moon, and zero when in deep space far away from all bodies imparting gravitational influence.

In the 20th century, the Newtonian concepts of gravitation were challenged by relativity. Einstein's principle of equivalence put all observers, accelerating in space far from gravitating bodies, or held in place against gravitation near such a body, on the same footing. This led to an ambiguity as to what exactly is meant by the "force of gravity" and (in consequence) by weight. The ambiguities introduced by relativity led, starting in the 1960s, to considerable debate in the teaching community as how to define weight for their students. The choice was a Newtonian definition of weight as the contact reaction-force against the force of gravity, for an object at rest on the ground, or an operational definition defined by the act of weighing. In the operational definition, weight becomes zero in conditions of weightlessness such as Earth orbit or free fall in vacuum. In such situations, the Newtonian view is that there remains a force due to gravity which is not measured (thus causing an apparent weight of zero), while the Einsteinian view is that there never does exist a measurable force due to gravity, even in everyday experience. Instead, weight and all sensation of weight are always produced by contact forces (push or pull) from the ground, or a scale. In free-fall, no force is measured simply because the force due to gravity is (still) never felt, and the floor (or the scale) now fails to exert the mechanical force that is what is always observed as "weight."

In everyday usage the term "weight" is commonly used to mean mass, which scientifically is an entirely different concept. On the surface of the Earth, the acceleration due to gravity (the "strength of gravity") is approximately constant; this means that the ratio of the weight force of a motionless object on the surface of the Earth to its mass is almost independent of its location, so that an object's weight force can stand as a proxy for its mass, and vice versa.

Read more about Weight:  History, Weight and Mass, Sensation of Weight, Measuring Weight, Relative Weights On The Earth and Other Celestial Bodies

Other articles related to "weight":

Boxing Training - Training - Weight
... Boxing, like several other fighting sports, categorizes its competitors into weight classes ... weigh-in so that they can be bumped down a weight class ... to three days before the weigh-in ceremony, in order to make weight for the fight ...
Egg White - Composition
... white is about two-thirds of the total egg's weight out of its shell, with nearly 92% of that weight coming from water ... The remaining weight of the egg white comes from protein, trace minerals, fatty material, vitamins, and glucose ...
Equine Forelimb Anatomy
... During locomotion, the forelimb functions primarily for weight-bearing rather than propulsion and supports the forehand of the horse ... support approximately 60% of the weight of the horse, and this pattern is carried over to locomotion, where the forelimb props the weight of the horse, while forward momentum is generated by the hind ... horse moves, increasing impulsion shifts the horse's weight to the hindquarters ...
Weight Function
... A weight function is a mathematical device used when performing a sum, integral, or average to give some elements more "weight" or influence on the result than other ... Weight functions can be employed in both discrete and continuous settings ...

Famous quotes containing the word weight:

    In a town-meeting, the great secret of political science was uncovered, and the problem solved, how to give every individual his fair weight in the government, without any disorder from numbers. In a town-meeting, the roots of society were reached. Here the rich gave counsel, but the poor also; and moreover, the just and the unjust.
    Ralph Waldo Emerson (1803–1882)

    as his weight wilts
    and he is on a porch
    that won’t hold my arms,
    or the legs of the race run
    forwards, or the film
    played backwards on his grandson’s eyes.
    Michael S. Harper (b. 1938)

    Now mark me how I will undo myself.
    I give this heavy weight from off my head,
    And this unwieldy sceptre from my hand,
    The pride of kingly sway from out my heart.
    With mine own tears I wash away my balm,
    With mine own hands I give away my crown.
    William Shakespeare (1564–1616)