Answer
$r=2.9ft$
Work Step by Step
As $a_{cp}=r\omega^2$
This can be rearranged as:
$r=\frac{a_{cp}}{\omega^2}$
We plug in the known values to obtain:
$r=\frac{3.0\times 9.81}{55 \frac{rev}{min}\times \frac{2\pi rad}{rev}\times \frac{1min}{60s}}$
$r=0.89m\times 3.281\frac{ft}{m}$
$r=2.9ft$
