The damping force ($F$) is directly proportional to velocity ($v$), which can be expressed as $F = -bv$, where $b$ is the damping constant (constant of proportionality). To find the units of $b$, we rearrange the equation: $b = \frac{F}{v}$ The SI unit of force ($F$) is Newton ($N$) or $kg \cdot m \cdot s^{-2}$. The SI unit of velocity ($v$) is $m \cdot s^{-1}$. Substituting these units: $\text{Unit of } b = \frac{kg \cdot m \cdot s^{-2}}{m \cdot s^{-1}} = kg \cdot s^{-1}$ Thus, the unit of the constant of proportionality is $kg \, s^{-1}$.