In studying Newton's laws, most teachers expect their students to be able to construct a free-body diagram that shows each of the forces that act on an object whose motion they want students to analyze. However, many students frequently cannot identify the correct number of forces that act on the object, or, having decided that a particular force should be added to the diagram, they cannot accurately identify what object exerts that force. In addition, many students have difficulty implementing Newton's third law and correctly identifying action-reaction pairs of forces as well as internal and external forces. In general students do not have an organized process for creating an abstract free-body diagram given a concrete physical situation. They make educated guesses without any way of knowing if what they have done is correct. The purpose of this article is to describe a tool that has been found to be valuable in helping students address each of these problems.

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A modeling method for high school physics instruction
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David Hestenes, “The Changing Role of Physics Departments in Modern Universities,” in Proceedings of the International Conference on Undergraduate Physics Education, Pt. 2, edited by Edward F. Redish and John S. Rigden (AIP, Woodbury, NY, 1997), p. 935.
David Hestenes, Thinking Physics for Teaching, edited by Carlo Bernardini et al. (Plenum Press, New York, 1995), p. 25.
David Hestenes presented to Phase 1 Leadership Modeling Workshop teachers, June 23, 1997. This talk is available online at
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