Where does force collide?

Whatever changes a body's momentum: a vector quantity equal to mass times acceleration, or more generally the rate of change of momentum. Newton fixed it in 1687 with the second law and made it symmetric with the third, every force is one half of a pair. Goldstein's textbook treatment shows the same content restated as potentials and Lagrangians, where force is the gradient of energy rather than a push. Forces add as vectors, so a body under balanced forces does not move at all, and a force does not need a mover with intentions.[1][2]

Unit

A body, or a pair of interacting bodies, at an instant

Recovery means

Not applicable, a force acts or it does not; the record is in the momentum

Measured by

Newtons (kg·m/s²); read from acceleration, deformation, or a balance

A directed tendency to move toward or away from a region of a person's or group's psychological field. Lewin borrowed the vector explicitly: in 1947 he described group standards as a quasi-stationary equilibrium held by driving forces pushing for change and restraining forces holding against it, and argued that removing a restraining force changes behaviour with less tension than adding a driving one. The 1951 collection set out the wider field theory. The force is defined by its direction and its target, not by a mass; Lewin never gave the magnitudes units, and force-field analysis today assigns them by judgement.[3][4]

Unit

A person or group's life space at a moment; a group standard under pressure

Recovery means

Refreezing, the equilibrium settles at a new level and holds

Measured by

Judged strength and direction of forces; change in a group standard after unfreezing

One of five structural features of an industry that together set the profit available to firms in it: rivalry among existing competitors, the threat of new entrants, the threat of substitutes, and the bargaining power of buyers and of suppliers. Porter fixed the list in 1979 in Harvard Business Review and restated it in 2008, insisting that the forces are properties of the industry and not of any firm, that they are not the same as growth rates or technology, and that strategy means positioning where the forces are weakest or reshaping them. A force here is a standing condition that lowers returns, and its strength is read from prices, margins and entry.[5][6]

Unit

An industry, over the medium term

Recovery means

Not applicable, the criterion is sustained return on invested capital above the cost of capital

Measured by

Industry profitability; concentration, entry barriers, switching costs, substitute price-performance

The set of people of working age who are either employed or actively looking for work; those neither working nor searching are out of the labour force. The definition is a statistical convention fixed by national agencies and the ILO's labour-statistics conferences, and it makes the participation rate (labour force over working-age population) a headline number alongside unemployment. Juhn and Potter's 2006 review shows how much rides on the boundary: the United States rate rose for half a century on women's entry and then fell as men's exits and population ageing took over. Nothing pushes here. The force is a count of people, and it exerts nothing.[7]

Unit

A working-age population in a reference week

Recovery means

Re-entry, a person who stopped searching resumes and is counted again

Measured by

Participation rate from household surveys; employed plus unemployed over population aged sixteen and over

Physical coercion, or the threat of it, applied by one political body to another or by a state to those within it. Weber's 1919 lecture defined the state as the community that successfully claims a monopoly of the legitimate use of physical force within a territory, so that every other use of force is licensed by the state or is a crime. International law took the other side of the same coin: Article 2(4) of the United Nations Charter forbids states to threaten or use force against one another, with self-defence and Security Council authorisation as the only exceptions. Franck's 1970 article asked whether the prohibition had survived its first quarter century. Force here is an act of will with a legal status, and the whole question is who may use it.[8][9]

Unit

A state, in relation to its subjects or to other states

Recovery means

Not applicable, the criterion is legitimacy or lawfulness of the act, not restoration

Measured by

Not measured; classified by legal category, lawful, unlawful, legitimate, illegitimate