Introductory physics is taught to several hundred thousand university students every year. It is seen as especially difficult by many and the failure rate is often high. A relevant question is whether one can increase the success rate among the weaker students? Retrieval practice is an established learning strategy with large benefits. However, as pointed out last year in this journal, hardly any systematic research has been done on retrieval practice in physics. Here we present a novel tool for retrieval practice in physics called the hierarchical principle structure for mechanics (HPSM). HPSM hierarchically organizes the essential principles, equations, and definitions for translational, rotational, and fluid mechanics, to emphasize meaningful connections. We investigated HPSM in a two-phase study. First, we present a randomized controlled experiment showing that 70 min of retrieval practice of HPSM had a very large effect on a declarative factual test compared to 70 min of problem study, d = 1.42. In the second phase, which was carried out the following year, we implemented distributed retrieval practice of HPSM in the first 15 min of 16 lectures. Although difficult to disentangle the effect from the lectures it was embedded in, distributed retrieval practice of HPSM seems to promote factual knowledge (r = 0.44) and better exam results for the weaker students (significant main and interaction effects).