Step 1 :Given that the air temperature $t$ is $5^\circ \mathrm{C}$ and the wind speed $v$ is $0.75 \mathrm{~m} / \mathrm{sec}$, we can see that $0 \leq v<1.79$. Therefore, we should use the first part of the formula, which is $W=t$.
Step 2 :Substituting the given values into the formula, we get: $W = t$
Step 3 :So, $W = 5^\circ \mathrm{C}$
Step 4 :Therefore, the wind chill for an air temperature of $5^\circ \mathrm{C}$ and a wind speed of $0.75 \mathrm{~m} / \mathrm{sec}$ is approximately $5^\circ \mathrm{C}$.
Step 5 :\(\boxed{W = 5^\circ \mathrm{C}}\)