This PhD thesis investigates pre-service physics teachers’ difficulties in understanding the relationship between Newton’s gravitational law and Kepler’s first two laws of planetary motion through Newton’s original formulation. The thesis encompasses two studies: one comparison study, one evaluation study. This is followed by a paper describing a comparison between Kepler’s and Newton’s approaches in determining the force laws and discussing some pedagogical implications of this comparison. The thesis builds on qualitative research design, where the think-aloud interview technique is given on ten pre-service physics teachers in one state university in Indonesia. The comparison study shows that by using geometrical derivation (Newton’s original formulation), pre-service physics teachers managed to comprehend the basic assumptions of the proof and the overall structure of the argument in deriving Kepler’s area law from the assumption of a central force. On the other hand, by using the analytical derivation, pre-service physics teachers could recall and apply the rules, but they failed to connect them to physical situations. The evaluation study demonstrates that one of the main difficulties in understanding Newton’s formulation in Proportion VI is reasoning with proportionality, and the participants could not distinguish it from equality. Furthermore, they had difficulties following a procedure to determine the force law once the orbit shape and sun’s position are given. Finally, the last paper of the thesis shows that by comparing Kepler’s and Newton’s formulations of their force laws, students could discuss two different approaches, namely, inductive and deductive, to formulate physics theories.