  We propose a simple nonlinear scaler displacement model to calculate the
distribution of effect created by a shear stress on a double stranded DNA
(dsDNA) molecule and the value of shear force $F_c$ which is required to
separate the two strands of a molecule. It is shown that as long as the force
pulls entire strand in the direction of its application the value of $F_c$
depends linearly on the length; the deviation from linear behaviour takes place
when part of a strand moves in opposite direction under the influence of force
acting on the other strand. The calculated values of $F_c$ as a function of
length of dsDNA molecules are in very good agreement with the experimental
values of Hatch et al (Phys. Rev. E $\bf 78$, 011920 (2008)).
