${?}^{****}(11pts{)}^{****}$ A particle moves along a straight line, with its position at time $t$ given by $s(t)=\frac{1}{\sqrt[\phantom{2}]{t}}-t,$ where $s(t)$ is in feet and $t$ is in minutes.

$($a$)$ Find the average velocity between $t=1$ and $t=4,$ but describe in a paragraph the physical meaning of your result in terms of the particle's motion. How would it differ if $s(t)$ were in meters instead of feet?

$($b$)$ Find the instantaneous velocity at $t=4$ and compare this with the average velocity. Explain how they differ conceptually.