Research focusing on concepts in Newtonian mechanics has been popular in the last twenty years. The reason for this is that it has been found that when students start a course in mechanics, many of them have beliefs that do not correspond with the concepts in Newton's theory, and to make students "unlearn" these beliefs can be very difficult.

In this essay a multiple-choice diagnostic test was used to examine the qualitative knowledge in elementary mechanics among students at the upper secondary school and at the university. The main purpose was to examine what concepts in classical mechanics are the most difficult ones for students to learn and to understand. But also other topics were examined; correlation between the physics grade and the result on the diagnostic test, difference between men and women, improvement on the diagnostic test after studying elementary mechanics and changes in the result on the diagnostic test in the last decade.

The results from the diagnostic test show that many students have difficulties to understand the concept of force. Many students believe that there has to exist a (net) force in the direction of motion, but they often use a mixture of this belief and the Newtonian force concept. The belief of "delayed forces" is also common; the students seem to believe that a force must not necessarily start (or stop) acting directly when it is applied (or removed).

In general, women have a lower result on the diagnostic test than men, also when comparing men and women who have the same grade from the physics A-course at the upper secondary school.

Physics education improves the result on the diagnostic test; the physics A-course at the upper secondary school seems to cause a clear improvement, but the improvement among men seems to be bigger than the improvement among women. A mechanics course at the university seems to cause only a small improvement on the diagnostic test.

No clear differences were found between the average results on the diagnostic test in these investigations and results from investigations in 1988 and 1989.