Deductive reasoning is a basic logic form used in scientific explanations and predictions. In dynamics, the process of finding the direction of force acting on a moving object, from the change of its motion, can be structured as a syllogism that is an elementary model of deduction. In this study, the syllogistic form of a scientific explanation task was used to help middle school students change their prior conceptions about force and motion. However, because the conclusion drawn from a syllogistic explanation task contradicted students' prior ideas, many rejected the conclusion or reached another conclusion without using deductive reasoning. From the preliminary interview using the syllogistic explanation task with eight students, we found four factors preventing students' use of deductive reasoning. In the main interview designed to remove these obstacles, it was observed that 26 of the 27 students could find the direction of force correctly by using deduction. Finally, implications for classroom teaching are discussed.