Unless otherwise specified, the domain of a function f is assumed to be the set of all real numbers x for which f(x) is a real number.

W(t)={172−32cos(πt6)10−15(t−6)2for0≤t≤6for6
The depth of a river at a certain point is modeled by the function W defined above, where W(t) is measured in feet and time t is measured in hours.

(a) Find W′(8). Using correct units, explain the meaning of W′(8) in the context of the problem.