Weights and Measures (CGPM) of 1901 to officially declare "The word weight denotes a quantity of the same nature as a force: the weight of a body is the product of its mass and the acceleration due to gravity", thus distinguishing it from mass for official usage. Relativity In the 20th century, the Newtonian concepts of absolute time and space were challenged by relativity. Einstein's equivalence principle put all observers, moving or accelerating, on the same footing. This led to an ambiguity as to what exactly is meant by the force of gravity and weight. A scale in an accelerating elevator cannot be distinguished from a scale in a gravitational field. Gravitational force and weight thereby became essentially frame-dependent quantities. This prompted the abandonment of the concept as superfluous in the fundamental sciences such as physics and chemistry. Nonetheless, the concept remained important in the teaching of physics. 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, choosing between a nominal definition of weight as the force due to gravity or an operational definition defined by the act of weighing. Definitions Several definitions exist for weight, not all of which are equivalent. Gravitational definition The most common definition of weight found in introductory physics textbooks defines weight as the force exerted on a body by gravity. This is often expressed in the formula , where W is the weight, m the mass of the object, and g gravitational acceleration. In 1901, the 3rd General Conference on Weights and Measures (CGPM) established this as their official definition of weight: This resolution defines weight as a vector, since force is a vector quantity. However, some textbooks also take weight to be a scalar by defining: The gravitational acceleration varies from place to place. Sometimes, it is simply taken to have a standard value of , which gives the standard weight. The force whose magnitude is equal to mg newtons is also known as the m kilogram weight (which term is abbreviated to kg-wt) Operational definition In the operational definition, the weight of an object is the force measured by the operation of weighing it, which is the force it exerts on its support. Since W is the downward force on the body by the centre of earth and there is no acceleration in the body, there