Answer
$(a)$ $M=k\frac{wh^2}{L}$
$(b)$ $k=400$
$(c)$ $M=12000lb$
Work Step by Step
$(a)$ According to the information given, we can write:
$M=k\frac{wh^2}{L}$
Where $k$ is constant of proportionality.
$(b)$
$w=4in$
$h=6in$
$L=12ft$
$M=4800lb$
$k$ is unknown
$4800=k\frac{4\times 36}{12}$
$12k=4800$
$k=400$
$(c)$
$L=10ft$
$w=3in$
$h=10in$
$k=400$
$M$ is unknown
$M=400\frac{3\times 100}{10}=12000lb$
