Solution
= Solution
The substrate contact area is the square bottom face, so the total <traction> force magnitude is stress times area:
$$
F=100\ \mathrm{N\,m^{-2}}\,(20\times10^{-6}\ \mathrm m)^2=4.0\times10^{-8}\ \mathrm N=\boxed{40\ \mathrm{nN}}.
$$
By <Newton's third law>, the force exerted by the substrate on the <biological cell> has the same magnitude and opposite direction to the cell's force on the substrate.