Where does pressure collide?

The normal force a fluid or solid exerts per unit area of a surface, a scalar with no direction of its own, measured in pascals. Pascal's treatise on the equilibrium of liquids, published in 1663, fixed the principle that carries his name: a pressure applied to a confined fluid is transmitted undiminished to every part of it, which is why a small piston can lift a large one. In a fluid at rest pressure acts equally in all directions and rises with depth; in a solid the same quantity is one component of the stress tensor. Nothing here is pushing on anyone. Pressure is a state of the medium.[1][2]

Unit

A point in a fluid or a surface element, at an instant

Recovery means

Not applicable, pressure is a state variable; the question is equilibrium, and whether the vessel holds

Measured by

Pascals, bar, atmospheres; manometers, piezoelectric and strain-gauge transducers

The pressure blood exerts on the walls of the arteries, reported as the peak during the heart's contraction over the trough between beats. Riva-Rocci's inflatable cuff of 1896 and Korotkoff's discovery in 1905 that the sounds heard over a compressed artery mark those two points made it a routine clinical number, still written in millimetres of mercury a century after the physics moved to pascals. The 2017 American guideline set the threshold for hypertension at 130 over 80 and made the measurement itself a matter of protocol: seated, rested, the right cuff, the average of readings. This is the physics meaning applied to one fluid in one vessel, and the only meaning on this page with a normal range.[3][4]

Unit

A person, at one measurement, or averaged across visits

Recovery means

Return to below the treatment threshold; sustained control on repeated measurement

Measured by

Millimetres of mercury; auscultatory or oscillometric cuff, ambulatory monitoring, intra-arterial line

Any factor that raises the importance of performing well on a particular occasion, a prize, an audience, a competition, being the one everyone depends on. Baumeister fixed the paradox in 1984: raising the stakes makes people attend to the mechanics of a skill they normally run automatically, and the skill degrades. Beilock and Carr showed in 2001 that this explicit monitoring, not distraction, is what breaks a well-learned motor skill, and that practising under self-focus inoculates against it. Pressure here is a property of the situation, manipulated by the experimenter and read off the drop in performance. Nobody is pushing; the performer pushes on themselves.[5][6]

Unit

A performer on one occasion, relative to their own baseline

Recovery means

Performance returns to baseline once the stakes are removed; training under pressure prevents the drop

Measured by

Performance decrement under incentive or audience; self-reported anxiety as a check on the manipulation

The influence a group exerts on a member to think or act as the group does, whether or not anyone says a word. Asch fixed the experimental meaning in 1951: a unanimous majority giving a plainly wrong answer about line lengths pulled a third of responses along with it, and the 1956 monograph mapped how the effect grows with majority size and collapses with one dissenter. Brown, Clasen and Eicher moved the word to adolescence in 1986 with the Peer Pressure Inventory, which asks teenagers how strongly their friends push them in each of several domains, and found the strongest reported pressures were toward spending time with peers and doing well at school, not toward misconduct. What separates this meaning from the physics is that pressure is perceived: it exists to the extent the person reports it.[7][8][9]

Unit

A person, in relation to a specific group, over a period

Recovery means

Not applicable, the criterion is independence: whether the person's judgement or behaviour survives the group

Measured by

Conformity rate on a judgement task; self-reported perceived peer pressure by domain

The activity of an organised group that makes claims on government on behalf of a shared interest, lobbying, testimony, campaign money, mobilised letters. Truman fixed the modern account in 1951, and refused the word: pressure group implied illegitimate coercion, so he wrote of interest groups, defined by a shared attitude and the claims they make on other groups through the institutions of government. Baumgartner and Leech showed in 1998 that half a century of research had produced hundreds of studies and no cumulative theory, largely because the field could not agree on what counted as pressure or how to measure it. Pressure here is exerted by an organisation on an institution, and the person being pressed is a legislator.[10][11]

Unit

An organised group, acting on a government or agency, over a policy episode

Recovery means

Not applicable, the criterion is influence: whether the claim changes the decision

Measured by

Lobbying registrations and spending, testimony counts, group membership; influence inferred from outcomes

A constraint placed on a purchase decision (a deadline, a quantity limit, a precondition) that changes how the buyer evaluates the offer and whether they decide at all. Inman, Peter and Raghubir showed in 1997 that restrictions on a deal raise its evaluation because buyers read the restriction as a signal that the deal is worth having. Dhar and Nowlis showed in 1999 that time pressure changes how much conflict a chooser feels between alternatives and so changes whether they defer the choice or make it. Pressure here is a lever the seller pulls, and the outcome of interest is not performance or conformity but a purchase.[12][13]

Unit

A consumer facing one purchase decision under a constraint

Recovery means

Not applicable, the criterion is the decision: purchase, defer, or walk away

Measured by

Deal evaluation, purchase intention, choice deferral rate, under manipulated time or quantity limits