In this letter, I explain an error which was identified to me in my use of the equation ##IMG##
[http://ej.iop.org/images/0031-9120/53/1/016502/pedaa968fieqn001.gif] {$v_n=e^nv_0$} in the first
method of measuring the coefficient of restitution of two colliding Newton’s cradle balls. In fact,
for this particular scenario, the correct equation is shown to be ##IMG##
[http://ej.iop.org/images/0031-9120/53/1/016502/pedaa968fieqn002.gif]
{$v_n=\left(\frac{1+e}{2}\right)^n v_0$} . However, it is further shown that, in this case, the
resulting error is small and the thesis of the paper remains valid.