Write an equation that expresses the following relationship. $w$ varies directly with the square of $u$ In your equation, use $k$ as the constant of proportionality.

Step 1 ：The question is asking for an equation that expresses the relationship between $w$ and $u$. The relationship is that $w$ varies directly with the square of $u$. This means that $w$ is proportional to $u^2$.

Step 2 ：In mathematical terms, this can be expressed as $w = ku^2$, where $k$ is the constant of proportionality.

Step 3 ：The equation that expresses the relationship is \(\boxed{w = ku^2}\).