Naive Physics

Naive physics refers to the commonsense beliefs that people hold about the way the world works, particularly with respect to classical mechanics. Being the oldest branch of physics, classical mechanics has priority because mechanical systems can be seen, whereas the motions relevant to other branches of physics are invisible. Because the motions of mechanical systems are both lawful and obvious, it is always intriguing to find instances in which people hold beliefs about mechanics that are not just underdeveloped but systematically wrong.

Jean PIAGET (1952, 1954) studied how young children acquire an understanding of the basic physical dimensions of the world and demonstrated that at young ages, children are systematically disposed to construe the world in biased ways. Interestingly, it was also found that adults often do not exhibit simple physical concepts that Piaget assumed they must have. A notable example of this is the water level problem introduced by Piaget and Inhelder (1956). When asked to indicate the surface orientation of water in a tilted container, about 40 percent of the adult population produce estimates that systematically deviate from the horizontal by more than 5 degrees (cf. McAfee and Proffitt 1991).

A large number of studies have found that adults express systematic errors in their reasoning about how objects naturally move in the world (Champagne, Klopher, and Anderson 1980; Clement 1982; Kaiser, Jonides, and Alexander 1986; McCloskey 1983; McCloskey, Caramazza, and Green 1980; McCloskey and Kohl 1983; Shanon 1976). An excellent introduction to this research can be found in McCloskey (1983), who dubbed this field of study "Intuitive Physics." The best-known example of these problems is the C-shaped tube problem, in which a participant is asked to predict the trajectory taken by a ball after it exits a C-shaped tube lying flat on a table (McCloskey, Caramazza, and Green 1980). The correct answer is that the ball will follow a straight trajectory tangent to the tube's curvature at the point of exit. About 40 percent of college students get this problem wrong and predict instead that the ball will continue to curve after exiting the tube.

There are two classes of explanations for these findings. The first supposes that people possess a general mental model that dictates the form of their errors (see MENTAL MODELS). One such proposal is that naive physics reflects an Aristotelian model of mechanics. Shanon (1976) found that many people reasoned, like Aristotle, that objects will fall at a constant velocity proportional to their mass. In his early writings, diSessa (1982) also argued that people display Aristotelian tendencies. McCloskey (1983) suggested that people's intuitive model resembled medieval impetus theory. By this account, an object is made to move by an impetus that dissipates over time. However, there are at least two problems with these mental model approaches to naive physics. The first is that people are not internally consistent (Cooke and Breedin 1994; diSessa 1983; Kaiser et al. 1992; Ranney and Thagard 1988; Shanon 1976). The same person will respond to different problems in a manner that suggests the application of different models. The second problem is that people are strongly influenced by the surface structure of the problem. Kaiser, Jonides, and Alexander (1986) found that people do not err on the C-shaped tube problem when the situation is put in a more familiar context. For example, no one predicts that water exiting a curved hose will continue to curve upon exit. Although people's dynamical judgments seem not to adhere to either implicit Aristotelian or impetus theories, this does not imply that they have no mental models applicable to natural dynamics. People may possess general models having some as yet undetermined structure, or their models may be domain specific.

The second type of EXPLANATION for people's systematic errors appeals to issues of problem complexity. Proffitt and Gilden (1989) proposed an account of dynamical event complexity that parsed mechanical systems into two classes. Particle motions are those that can be described mathematically by treating the moving object as if it were a point located at its center of mass. Extended-body motions are those contexts in which the object's mass distribution, size, and orientation influence its motion. As an example, consider a wheel. If the wheel is dropped in a vacuum, then its velocity is simply a function of the distance that its center of mass has fallen. Ignoring air resistance, freefall is a particle motion. On the other hand, if the wheel is placed on an inclined plane and released, then the wheel's shape -- its moment of inertia -- is dynamically relevant. This is an extended-body motion context. People reason fairly well about particle motion problems but not extended-body motion ones. In addition, people also err when they misrepresent a particle motion as being an extended-body motion as, for example, in the C-shaped tube problem. In doing so, they attribute more dimensionality to the problem than is actually there.

Given that people often predict that events will follow unnatural courses -- for example, that a ball exiting a C-shaped tube will persist to follow a curved path -- it is interesting to ask what would happen if they actually saw such an event occur. Would it look odd or natural? Kaiser and Proffitt (Kaiser, Proffitt, and Anderson 1985; Kaiser et al. 1992; Proffitt, Kaiser, and Whelan 1990) found that when presented with animations of particle motion problems, people judged their own predictions to be unnatural and selected natural motions as appearing correct. For example, when contrived animations were presented to people who drew curved paths on the paper-and-pencil version of the problem, these people reported that balls rolling through a C-shaped tube and continuing to curve upon exit looked very odd, whereas straight paths appeared natural. Animations, however, did not evoke more accurate judgments for extended-body motions (Proffitt, Kaiser, and Whelan 1990). For example, Howard (1978) and McAfee and Proffitt (1991) found that viewing animations of liquids moving to nonhorizontal orientations in tilting containers did not evoke more accurate judgments from people prone to err on this problem.

Adults' naive conceptions about how the world works appear to be simplistic, inconsistent, and situation-specific. However, recent research with infants suggests that a few core beliefs may underlie all dynamical reasoning (see INFANT COGNITION). Baillargeon (1993) and Spelke et al. (1992) have shown that, by around 2 1/2 months of age, infants can reason about the continuity and solidity of objects involved in simple events. Other physical concepts, such as gravity and inertia, do not seem to enter infants' reasoning until much later, around 6 months of age (Spelke et al. 1992). Spelke et al. proposed the intriguing notion that continuity and solidity are core principles that persist throughout the development of people's naive physics.

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-- Dennis Proffitt

References

Baillargeon, R. (1993). The object concept revisited: New directions in the investigation of infants' physical knowledge. In C. E. Granrud, Ed., Carnegie Symposium on Cognition: Visual Perception and Cognition in Infancy. Hillsdale, NJ: Erlbaum, pp. 265-315.

Champagne, A. B., L. E. Klopher, and J. H. Anderson. (1980). Factors influencing the learning of classical mechanics. American Journal of Physics 48:1074-1079.

Clement, J. (1982). Students' preconceptions in introductory mechanics. American Journal of Physics 50:66-71.

Cooke, N. J., and S. D. Breedin. (1994). Constructing naive theories of motion on the fly. Memory and Cognition 22:474-493.

diSessa, A. (1982). Unlearning Aristotelian physics: A study of knowledge-based learning. Cognitive Science 6:37-75.

diSessa. A. (1983). Phenomenology and the evolution of intuition. In D. Gentner and A. L. Stevens, Eds., Mental Models. Hillsdale, NJ: Erlbaum, pp. 15-33.

Howard, I. (1978). Recognition and knowledge of the water-level problem. Perception 7:151-160.

Kaiser, M. K., J. Jonides, and J. Alexander. (1986). Intuitive reasoning about abstract and familiar physics problems. Memory and Cognition 14:308-312.

Kaiser, M. K., D. E. Proffitt, and K. A. Anderson. (1985). Judgments of natural and anomalous trajectories in the presence and absence of motion. Journal of Experimental Psychology: Human Perception and Performance 11:795-803.

Kaiser, M. K., D. R. Proffitt, S. M. Whelan, and H. Hecht. (1992). Influence of animation on dynamical judgments. Journal of Experimental Psychology: Human Perception and Performance 18:384-393.

McAfee, E. A., and D. R. Proffitt. (1991). Understanding the surface orientation of liquids. Cognitive Psychology 23:669-690.

McCloskey, M. (1983). Intuitive physics. Scientific American 248:122-130.

McCloskey, M., A. Caramazza, and B. Green. (1980). Curvilinear motion in the absence of external forces: Naive beliefs about the motion of objects. Science 210:1139-1141.

McCloskey, M., and D. Kohl. (1983). Naive physics: The curvilinear impetus principle and its role in interactions with moving objects. Journal of Experimental Psychology: Learning, Memory, and Cognition 9:146-156.

Piaget, J. (1952). The Origins of Intelligence in Childhood. New York: International Universities Press.

Piaget, J. (1954). The Construction of Reality in the Child. New York: Basic Books.

Piaget, J., and B. Inhelder. (1956). The Child's Conception of Space. London: Routledge and Kegan Paul.

Proffitt, D. R., and D. L. Gilden. (1989). Understanding natural dynamics. Journal of Experimental Psychology: Human Perception and Performance 15:384-393.

Proffitt, D. R., M. K. Kaiser, and S. M. Whelan. (1990). Understanding wheel dynamics. Cognitive Psychology 22:342-373.

Ranney, M., and P. Thagard. (1988). Explanatory coherence and belief revision in naive physics. In Proceedings of the Tenth Annual Conference of the Cognitive Science Society. Hillsdale, NJ: Erlbaum, pp. 426-432.

Shanon, B. (1976). Aristotelianism, Newtonianism, and the physics of the layman. Perception 5:241-243.

Spelke, E. S., K. Breinlinger, J. Macomber, and K. Jacobson. (1992). Origins of knowledge. Psychological Review 99:605-632.

Naive Physics

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References

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