Sources of Difficulty in Understanding Newtonian Dynamics*

A series of force and motion problems was presented to 40 high school science students. The results revealed that the majority of the students only partially understood the formalisms of Newtonian physics. Many had difficulty determining the effect that a force would have on an object' speed of motion and many neglected to take into account thecurrent speed of motion when attempting to predict how a force would alter an obiect's direction of motion. The students' justifications for their erroneous answers suggest that they utilize diverse, and often inconsistent, ideas when solving force and motion problems. These include (a) faulty beliefs about force and motion which have been derived from living in a world where unseen frictional forces operate; (b) partially understood examples from physics class; and (c) knowledge of the properties of scalar arithmetic, some of which do not transfer to vector arithmetic. The students thus employ components of their knowledge, which seem relevant to the solution of force and motion problems, but which often have properties that conflict with the implications of Newton's laws of motion.