  In classical physics, calculating the slack of a hanging chain is a problem
that has attracted interest. This study aims to solve this problem through
experiment and theory. When the length and distance of both the ends of a
hanging chain are given, the length of slack can be expressed by a Lambert W
function or an irrational function within a certain distance. Herein, a simple
observation of the slack is presented. The result obtained is one of the
applications of the Lambert W function in the field of physics. Though the
shape of a hanging chain is well-known to be a catenary, the calculation of
slack possesses richer aspects of mathematical physics.
