Differential and Integral Calculus by Feliciano and Uy, Exercise 1.1 Problem 6

The stiffness of a beam of rectangular cross section is proportional to the breadth and the cube of the depth. If the breadth is 20 cm, express the stiffness as a function of the depth.


Let S=stiffness, b=breadth, and d=depth

\displaystyle S=bd^{3\:}

\displaystyle S=20d^3