Gravitational Force (Weight)
Forces and Motion

A brief history of the concepts of mass and weight

Stories from Physics for 11-14 14-16 IOP RESOURCES

The term ‘mass’ is derived from the Latin massa meaning a lump of dough or paste. In many ancient cultures, despite standardisation of units such as length and volume, different units of weights were used for different goods and measures varied from place to place. Therefore, it has been argued that weight was not conceived of as a unified concept but rather as a property of individual objects, like colour or smell. This view may have been the cause of Aristotle’s perception that heavy objects fall faster than light ones. Ancient Greek philosophers thought weight derived from centrifugal forces whereas Romans thinkers believed weight resulted from a natural tendency of objects to fall. Aristotle argued that the heavy elements, earth and water, have gravity, a tendency to move to the centre of the universe, whereas light elements, air and fire, have levity, a tendency to move away from the centre of the universe. Galileo, philosophers of science have argued, understood the concept of inertial mass without ever offering an explicit definition of the term.

Newton used the term pondus which translates to ‘weight’ to refer to a measure of matter but was aware of a distinction between quantity of matter and gravitational force. He reported that, because of ‘the want of a suitable word’ he would ‘represent and designate quantity of matter by weight’ though he clarifies that by weight ‘I mean the quantity or amount of matter being moved, apart from considerations of gravity, so long as there is no question of gravitating bodies’. Newton defined mass in the following way: ‘The quantity of matter is the measure of the same, arising from its density and bulk conjointly’. Ernst Mach suggested that Newton’s definition was circular and proposed an argument that mass is a concept constructed simply to make sense of the motion of objects.

While contemporary textbooks typically define mass either as the quantity of matter or as an object’s ability to resist changes in motion, Hecht argues that it is impossible to create a completely operational definition of mass as valid measurement is not practically possible.


Gravitational Force (Weight)
is a feature of the Rigid Body Model
is a special case of Force
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