We make some crucial remarks about the recent presentation by Fredlund et al (2015 Eur. J. Phys. 36
[http:// http://dx.doi.org/10.1088/0143-0807/36/5/055002] 055002 ) considering the tutorial problem
raised therein. After working out the velocity of the electron (we also included the role of image
charges or induced charges) as it strikes the (conducting) metal sphere, we found the velocity value
is already near the relativistic regime. The latter then encounters the open issue; to obtain a
classical equation of motion of a point charge for which Yaghjian (2008 Phys. Rev. E 78 [http://
http://dx.doi.org/10.1103/physreve.78.046606] 046606 ) has mentioned the following difficulty: the
electrostatic energy of formation and thus the electrostatic mass of a point charge is infinite.