Answer
$9.0Km$
Work Step by Step
As we know that
$E=mgh$
$E=(0.55\times 10^{-3})(9.81)(0.28)=1.51\times 10^3J$
Now $P=\frac{E}{t}=\frac{1.51\times 10^3}{1.5}=1.01\times 10^{-3}W$
We also know that
$I_{\circ}=\frac{P}{4\pi r^2}$
This can be simplified and rearranged as:
$r=\sqrt{\frac{P}{4\pi rI_{\circ}}}$
We plug in the known values to obtain:
$r=\sqrt{\frac{1.01\times 10^{-3}}{4\pi(10^{-12})}}=9.0Km$
