Answer
$640\;lb$.
Work Step by Step
The given values are
$F_s=40\;lb$
$r_l=28\;in.$
$r_s=7\;in.$
Formula
$\Rightarrow \frac{F_l}{F_s}=\frac{r^2_l}{r^2_s}$
Substitute all the values.
$\Rightarrow \frac{F_l}{40\;lb}=\frac{(28\;in.)^2}{(7\;in.)^2}$
Simplify.
$\Rightarrow F_l=(40\;lb)\frac{(28\;in.)^2}{(7\;in.)^2}$
$\Rightarrow F_l=640\;lb$
Hence, the $F_l$ is $640\;lb$.
