Answer
$400\;lb$.
Work Step by Step
The given values are
$F_l=6400\;lb$
$r_l=16\;in.$
$r_s=4\;in.$
Formula
$\Rightarrow \frac{F_l}{F_s}=\frac{r^2_l}{r^2_s}$
Substitute all the values.
$\Rightarrow \frac{6400\;lb}{F_s}=\frac{(16\;in.)^2}{(4\;in.)^2}$
Simplify.
$\Rightarrow (6400\;lb)\frac{(4\;in.)^2}{(16\;in.)^2}=F_s$
$\Rightarrow F_s=400\;lb$
Hence, the $F_s$ is $400\;lb$.
