The purpose of this work is to study the effectiveness of our introductory mechanics instructions in changing students' misconceptions about Newtonian concepts. We gave the force concept inventory (FCI) test to 341 of our students twice, before and after instructions. The data were analyzed using a method first reported by Martín-Blas "et al" (2010 "Eur. J. Eng. Educ." 35 597-606). We examined all of the known misconceptions found in the original paper by Hestenes "et al" (1992 "Phys. Teach." 30 141-51) and determined the dominant ones from students' incorrect answers for each of the 30 questions in the inventory. By comparing the dominant misconceptions held by our students in the pre- and post-tests, we were able to determine which ones were unchangeable by instructions and students couldn't overcome. Results indicate some overall improvement in students' performance but also the existence of very resistive to change misconceptions that were not affected by instructions.