The radius, $r$, in centimeters, of a melting snowball is given by $r = 45-3.5t$, where $t$ is the time in hours. The snowball is spherical with volume $V = \frac{4}{3} \pi r^3 \ \mathrm{cm}^3$. Find a formula for $V = f(t)$, the volume of the snowball as a function of time.

$V = f(t) =$ help (formulas) with help (units)
(Be sure to include units!)