Chapter 13 - Section 13.3 - Tangent Lines and Derivatives - 13.3 Exercises - Page 922: 37

$V=-24\frac{ft}{s} $ direction is downwards

$v=\dfrac {\partial y}{\partial t}=\dfrac {\partial }{\partial t}\left( 40t-16t^{2}\right) =40-32t\Rightarrow t=2s\Rightarrow v\left( 2\right) =40-32\times 2=-24\dfrac {ft}{s}$ $-$ sign means ball moving downward at $t=2s$