Answer
a. $32\ ft/sec$
b. $-32\ ft/sec$
c. upward motion, downward motion.
Work Step by Step
a. Using the given equation, we have $s(0)=80\ ft$ and $s(2)=-16(2)^2+64(2)+80=144\ ft$. Thus the average velocity for this time interval can be found as $\bar v_1=\frac{144-80}{2-0}=32\ ft/sec$
b. We have $s(4)=-16(4)^2+64(4)+80=80\ ft$. Thus the average velocity for this time interval can be found as $\bar v_2=\frac{80-144}{4-2}=-32\ ft/sec$
c. The result from part-a is positive, indicating an upward motion. The result from part-b is negative, indicating a downward motion.
