Design of Everyday Things - Norman, Don [25]
ARISTOTLE’S NAIVE PHYSICS
For example, Aristotle thought that moving objects kept moving only if something kept pushing them. Today’s physicist says nonsense: a moving object continues to move unless some force is exerted to stop it. This is Newton’s first law of motion, and it contributed to the development of modern physics. Yet anyone who has ever pushed a heavy box along a street or, for that matter, hiked for miles into the wilderness, knows that Aristotle was right: if you don’t keep on pushing, the movement stops. Of course, Newton and his successors assume the absence of friction and air. Aristotle lived in a world where there was always friction and air resistance. Once friction is involved, then objects in motion tend to stop unless you keep pushing. Aristotle’s theory may be bad physics, but it describes reasonably well what we can see in the real world. Think about how you might answer the following questions.
1. I take a pistol and, carefully aiming it on a level, horizontal line, I fire a bullet With my other hand, I hold a bullet so that the bullet in the pistol and the one in my hand are exactly the same distance from the ground I drop the bullet at the same instant as I fire the pistol. Which bullet hits the ground first?
2. Imagine someone running across a field carrying a ball As you watch, the runner drops the ball Which path (a, b, or c in figures 2.1) does the ball take as it falls to the ground?2
The physicist says the answer to the bullet problem is trivial: both bullets hit the ground at the same time. The fact that one bullet is traveling horizontally very rapidly has absolutely no effect on how fast it falls downward. Why should we accept that answer? Shouldn’t the speeding bullet develop some lift—sort of like an airplane—so that it will stay up a bit longer because it is kept up by the air? Who knows? The theory of physics is based upon a situation where there is no air. The popular misconception is that the pistol bullet will hit the ground long after the dropped bullet; yet this naive view doesn’t seem so strange.
2.1 A Running Man Drops a Ball. Which path does the ball take as it falls to the ground, path A, B, or C? When this question was asked of sixth-grade students in Boston schools, only 3 percent answered A, the right answer; the others were evenly divided between B and C. Even high school students did not do well: of forty-one students who had just studied Newtonian mechanics for a month and a half, only 20 percent got the right answer; the others were almost equally divided between B and C. (The study was performed by White & Horwitz, 1987. The figure is reprinted from lntuitive Physics by McCloskey. Copyright © 1983 by Scientific American, Inc. All rights reserved.)
In the case of the falling ball, our prediction is that the ball will drop straight down. In fact, the falling ball follows trajectory A (figure 2.1). As it is carried by the runner, it is set into horizontal motion. It then maintains the same forward speed upon being released, even as it also falls to the ground.3
Naive physics—and naive views of psychology and other fields—are often sensible, even if wrong. But at times they can get us into trouble. Yet we must have a way to digest the unfamiliar, for people are explanatory creatures.
PEOPLE AS EXPLANATORY CREATURES
Mental models, our conceptual models of the way objects work, events take place, or people behave, result from our tendency to form explanations of things. These models are essential in helping us understand our experiences, predict the outcomes of our actions, and handle unexpected occurrences. We base our models on whatever knowledge we have, real or imaginary,