1. The problem asks why we write $s(t) = 0$ in certain contexts. 2. Typically, $s(t)$ represents a position function depending on time $t$. 3. Writing $s(t) = 0$ means the position at time $t$ is zero, often indicating a reference point or origin. 4. This is used to set initial conditions or to find when an object returns to the starting point. 5. For example, if $s(t)$ measures displacement from a fixed point, $s(t) = 0$ means the object is exactly at that point. 6. This helps solve equations for $t$ when the object crosses or reaches the origin. 7. In summary, $s(t) = 0$ is a way to specify or find times when the position is at the reference location.