Comment on 'Approaching the brachistochrone using inclined planes-striving for shortest or equal travelling times'

In a recent article, Theilmann (2016 Phys. Ed. 52 015009) analyses experimental results for a ball rolling along an inclined surface. While the article has some merit, it also contains fundamental flaws. The author makes the assumption that kinetic friction for translational motion is analogous to that of rotational motion; this assumption is incorrect. Furthermore, there is an inconsistency between data presented in the paper and the conclusion of the ball sliding during the experiment. The article contains other smaller inaccuracies.