Interpreting Function Notation, Graphs, and Context

You just got home from college and your dog is running towards you. The speed of your dog in meters per second($m/s$) can be modeled as: $$S=2w+8$$ Where: $$\begin{align} S&= \text{ Speed of dog}\\ w&= \text{ Number of weeks you were away}\end{align}$$

What is the independent variable?