Abstract: The energy method is often used to analyze the stresses in members under impact loading in mechanics of materials. In the process of teaching, it is found that students are not clear about the application range of the theory and neglect or misunderstand the significance of the fundamental hypothesis in the theory. This paper takes a bar under axial impact as an example. By using the theory of elastodynamics and finite element method, the actual dynamic response in the bar is illustrated. The theoretical predictions from the energy method, elastodynamics, and finite element method for different impact mass ratios are compared and explained in order to help students understand the importance of the fundamental hypothesis in the analysis of stress of members under impact loading by using the energy method in mechanics of materials, which enables students to establish simple and reliable mechanical models to solve engineering problems.