The worm-like chain model is a simple continuum model for the statistical mechanics of a flexible
polymer subject to an external force. We offer a tutorial introduction to it using three approaches.
First, we use a mesoscopic view, treating a long polymer (in two dimensions) as though it were
made of many groups of correlated links or “clinks,” allowing us to calculate its average extension
as a function of the external force via scaling arguments. We then provide a standard statistical
mechanics approach, obtaining the average extension by two different means: the equipartition
theorem and the partition function. Finally, we work in a probabilistic framework, taking
advantage of the Gaussian properties of the chain in the large-force limit to improve upon the
previous calculations of the average extension