billiard ballsbilliard balls
Collisions abound on a billiards table. Billiard balls collide with each other and with the sides of the table. These collisions result in changes in the momentum of the billiard balls. They slow down, speed up and change directions. All three of these changes require a force acting upon the ball for some amount of time. This force acting over time is known as an impulse. An impulse causes a momentum change. Not only does the impulse cause a momentum change, it is also equal to the momentum change.
In social models, billiard ball model refers to the comparison of movements and collisions of humans to that of the movement and collisions billiard balls, often done in order to gain insight into the similarities and or differences between the two types of movement, and or to discuss or theorize about concepts such as purpose, meaning, morality, consciousness, intention, and or causality, among others philosophical inquiries.

In 1898, Scottish physical chemist William Ramsay, in his discussion of the kinetic theory, compared the collisions of football players, defined as human molecules, to the collisions of gas molecules: [1]

“I find, in my own case, that it helps greatly to a clear understanding of a concept if a mental picture can be called up which will illustrate the concept, if even imperfectly. Some such picture may be formed by thinking of the motions of the players in a game of football. At some critical point in the game, the players are running, some this way, some that; one has picked up the ball and is running with it, followed by two or three others; while players from the opposite side are slanting towards him, intent upon a collision. The backs are at rest, perhaps; but, on the approach of the ball to the goal, they quicken into activity, and the throng of human molecules is turned and pursues an opposite course. The failure of this analogy to represent what is believed to occur in a gas is that the players’ motion is directed and has purpose; that they do not move in straight lines, but in any curves which may suit their purpose; and that they do not, as two billiard-balls do, communicate their rates of motion to the other by collision. But, making such reservations, some idea may be gained of the encounters of molecules by the encounters in a football-field.”

In football, collisions are going to happen during every play. The offensive and defensive players approach each other momentum. They each have their mass in motion. Momentum, being a vector quantity, has a direction. The direction of the momentum is in the direction that the player is moving. So in effect, it is the goal of each player to gather more momentum in the direction opposite their opponent prior to the collision. During the collision, both objects change their momentum. The momentum changes of the two objects are equal in magnitude and opposite in direction. So the northward-moving play may lose 300 kg•m/s of northward momentum. If so, then the southward-moving player loses 300 kg•m/s of southward momentum. This is referred to as momentum conservation. [2]

Here we see the terms "goals" and "purpose" interjected into the discussion of the description of the motion of the humans as compared to the motion of the balls, which are comparison topics found in recent treatises, such as by Cuban-born American philosopher Alicia Juarrero (1999) and American anthropological neuroscientist Terrence Deacon (2011), in attempts to bridge the conceptual gap between the materialism-physicalism world and the human consciousness/mind teleology-conceptualized world by making amends to thermodynamics. [3]

The "billiard ball vs. human" comparison is similar to the classical "rock vs. human" comparison, albeit slightly more complex in that the former takes "collisional" motion, not just motion alone, into context.

See also
Chemistry set model
Social cannon ball model

1. Ramsay, William. (1898). “The Kinetic Theory of Gases and Some of its Consequences” (human molecules, pg. 685). The Contemporary Review, 74: 681-91.
2. Momentum and Collisions (Flickr) – The Physics Classroom.
3. (a) Juarrero, Alicia. (1999). Dynamics in Action: Intentional Behavior as a Complex System (billiard ball, 7+ pgs). MIT Press.
(b) Deacon, Terrence W. (2011). Incomplete Nature: How Mind Emerged from Matter (billiard ball, 5+ pgs). W.W. Norton & Co.

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