Naive physics: even among college students, old beliefs linger

Medieval Europeans thought moving bodies acquired an "impetus" that kept them going in the absence of external forces until this "impetus" gradually dissipated. They were wrong -- moving bodies keep moving indefinitely at constant speed in a straight line when no force acts upon them. But in spite of Newton, Einstein, and astronauts who correctly plotted courses to the moon, the medieval belief seems to linger.

It may even be reflected in the way some college students who have studied physics think about the subject.

Michael McCloskey, Alfonso Caramazza, and Bert Green of Johns Hopkins University report a study indicating that many students who have completed one or more physics courses, as well as many students with no such instruction, fail to grasp the fundamental principles of mechanics. This is not simply ignorance of how things move, but a firm and widely shared preconception of how they ought to move -- a kind of naive physics with "laws of motion" reminiscent of the medieval view.

In saying this, the psychologists are generalizing from the results of an experiment with 47 students recently described in Science. Of these, 15 had no formal physics education, 22 had completed one high school physics course, and 10 had finished one or more college physics courses.

They were given a set of diagrams in which balls moved along curved paths. They were asked to show how the balls would move when the forces causing them to travel along curves were removed. In each case, the correct answer would be motion in a straight line. But a significant number of the students didn't think of it that way.

For example, on a diagram where a ball went through a tube curved in the form of a "C," 33 percent of the students showed it emerging from the tube and continuing along a curved path. This type of error reached 51 percent when the diagram involved a tube bent into a spiral. Also, 30 percent of those tested thought a ball whirled on the end of a string would continue.

This was a consistent mistake. It exhibited a shared misconception among large percentages of those tested. Interviews with some of them indicated they thought the curve of the tubes or the pull of the string would impart a carving "momentum" to the ball that would eventually dissipate. This echoes the medieval belief in an"impetus."

As the Johns Hopkins scientists point out, educators need to do more than try to teach the facts of a subject like physics. They also need to take account of any strong preconceptions students bring to the subject. Otherwise, they may only teach a "new terminology for expressing . . . erroneous beliefs."

Beyond this, their results hint at the persistence of old beliefs within a culture (in this case, Western culture) long after a different world view has supposedly displaced them.

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