A particle moves according to a law of motion $s = f(t)$, $t \ge 0$, where $t$ is measured in seconds and $s$ in feet.
$f(t) = 0.01t^4 - 0.05t^3$
(a) Find the velocity at time $t$ (in ft/s).
$v(t) = \boxed{\phantom{0.04t^3 - 0.15t^2}}$
(b) What is the velocity after 2 second(s)?
$v(2) = \boxed{\phantom{-0.34}}$ ft/s
(c) When is the particle at rest?
$t = \boxed{\phantom{0}}$ s (smaller value)
$t = \boxed{\phantom{3.75}}$ s (larger value)