The main goal of the research is to determine how certain teaching instruction methods affect the achievement and mental efforts of high school students needed for learning Fluid Mechanics topic in Physics. Determining mental effort or cognitive load as a wider concept helps obtain important data, which can be used to identify teaching instruction menthods, which result in higher performance and motivation. This research is aimed to examine the efficiency of three approaches to teaching physics, which are most common in the Republic of Serbia. These are: an approach based on the use of laboratory inquiry-based experiments (LIBE), an approach based on the use of interactive computer-based simulation (ICBS) and a traditional teaching approach (TA). The article describes an experimental study conducted with two experimental and one control groups. The research was conducted on a sample of six high school classes in a gymnasium with advanced study in Natural Science and Mathematics in Novi Sad, Republic of Serbia. The total sample count was 187 students (mean age 16 years). The main conclusions of the research are that there is a causal link between the teaching instruction method applied and the achievement, or the self-perceived mental effort, of a student. Students, who were learning the teaching content through LIBE or ICS approach, have achieved better results in the knowledge test and estimatd their mental effort to be lower compared to the students, who were learning the same content through traditional teaching approach applied. The reasearch also showed, that LIBE or ICBS teaching approaches achieve higher levels of instructional efficiency and instructional involvement compared to the traditional teaching approach.