= Solution
The <AFM> pulls a tethered molecule through its <cantilever>. Deflection, multiplied by a calibrated <cantilever> <spring constant>, measures <force>; support motion must be corrected for that deflection and any linker compliance to obtain molecular extension. Initially folded structure can resist extension, then contacts break and more <polypeptide> <polymer contour length> becomes available. The readable trace rises with increasing extension, with fluctuations and a strongly rising large-extension part. It should not be interpreted as a direct image of individual <amino acids> or as establishing a unique microscopic unfolding pathway.
An ideal inextensible <freely jointed chain> with the stated segment count and length has
$$
\boxed{L_c=Nb=35(0.4\,\mathrm{nm})=14\,\mathrm{nm}.}
$$
Its mean extension is strictly below $14\,\mathrm{nm}$ at every finite <force>. Thus the quoted $15\,\mathrm{nm}$ maximum does not establish ideal <FJC> behaviour: if both numbers are taken literally, it exceeds the model's <polymer contour length> by about $7\%$. Finite segment stretch, inaccurate effective segment parameters, additional linkers or an extension-zero/calibration error can account for such a difference. If the numerical descriptions are approximate, the discrepancy may be within their uncertainty; even exact agreement with a <polymer contour length> would not prove freely jointed statistics. Low-<force> fluctuations and the full <force>–extension relation are separate tests.
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