I would like to discuss here some points from the article Martin Swaczyna Several examples of nonholonomic mechanical systems Communications in Mathematics, Vol. 19 (2011), No. 1, 27--56 This article is available at https://eudml.org/doc/196963 Let us open page 37: This is a classical problem. Please pay your attention to that the author does not assume the value of dog's velocity to be constant. Below he derives this fact as a consequence from equations of dog's motion. I find that strange and physically incorrect. Any opinions?