En física clásica, el peso es una medida de la fuerza gravitatoria que actúa sobre un objeto. El peso equivale a la fuerza que ejerce un cuerpo sobre un punto de apoyo, originada por la acción del campo gravitatorio local sobre la masa del cuerpo. Por ser una fuerza, el peso se representa como un vector, definido por su módulo, dirección y sentido, aplicado en el centro de gravedad del cuerpo y dirigido aproximadamente hacia el centro de la Tierra. Por extensión de esta definición, también podemos referirnos al peso de un cuerpo en cualquier otro astro (Luna, Marte,...) en cuyas proximidades se encuentre. La magnitud del peso de un objeto, desde la definición operacional de peso, depende tan solo de la intensidad del campo gravitatorio local y de la masa del cuerpo, en un sentido estricto. Sin embargo, desde un punto de vista legal y práctico, se establece que el peso, cuando el sistema de referencia es la Tierra, comprende no solo la fuerza gravitatoria local, sino también la fuerza centrífuga local debido a la rotación de la Tierra; por el contrario, el empuje atmosférico no se incluye, ni ninguna otra fuerza externa.

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In science and engineering, the weight of an object is usually taken to be the force on the object due to gravity. Weight is a vector whose 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. 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, and about one-sixth as much on the Moon. In this sense of weight, a body can be weightless only if it is far away (in principle infinitely far away) from any other mass. Although weight and mass are scientifically distinct quantities, the terms are often confused with each other in everyday use (i.e. comparing and converting force weight in pounds to mass in kilograms and vice versa). There is also a rival tradition within Newtonian physics and engineering which sees weight as that which is measured when one uses scales. There the weight is a measure of the magnitude of the reaction force exerted on a body. Typically, in measuring an object's weight, the object is placed on scales at rest with respect to the earth, but the definition can be extended to other states of motion. Thus, in a state of free fall, the weight would be zero. In this second sense of weight, terrestrial objects can be weightless. Ignoring air resistance, the famous apple falling from the tree, on its way to meet the ground near Isaac Newton, is weightless. Further complications in elucidating the various concepts of weight have to do with the theory of relativity according to which gravity is modelled as a consequence of the curvature of spacetime. In the teaching community, a considerable debate has existed for over half a century on how to define weight for their students. The current situation is that a multiple set of concepts co-exist and find use in their various contexts.

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