The atomic force microscope can be used to forcibly unfold and extend single polypeptide chains. The resulting force versus distance curves have been widely interpreted to arise from the loss of entropy that the unfolded polypeptide chain experiences as it is extended. Here, we have used Monte Carlo simulations of unfolded polypeptide chains to examine the average distance between the ends of a polypeptide chain as a function of the force that pulls these ends apart. We examine two types of experiments: (a) A rigid force-sensor (bead-type) experiment: The chain is subjected to a constant stretching force f and the resulting chain extension is measured. (b) A flexible force-sensor (cantilever-type) experiment: The force is measured by the deflection of a cantilever that is attached to one end of the chain. The total length of the chain plus the displacement of the cantilever is fixed. In case (b), in the limit of a large cantilever force constant, the entropic force f is related to the free energy of the chain F(r) constrained to have the end-to-end distance r by the usual thermodynamic relationship: f=dF/dr. However in case (a) this relationship is invalid. The reason of its failure is that large fluctuations in the end-to-end distance r cannot be neglected at the single molecule level and so macroscopic thermodynamics relationships cannot be used. Thus the two types of experiments measure different force extension curves f(r). We compute the force extension curves for a model of a polypeptide chain in each case and find that they are significantly different. We further discuss implications of our findings with regard to the results of cantilever-type unfolding experiments.