Step 1: Relating Acceleration to Velocity
Given that acceleration \( a \) is proportional to velocity \( v \), we have:
\[
a = k v
\]
Since acceleration is the rate of change of velocity, we can write:
\[
\frac{dv}{dt} = k v.
\]
Step 2: Formulating the Differential Equation
Rearrange the equation to separate the variables:
\[
\frac{dv}{v} = k \, dt.
\]
This is the desired first-order differential equation.