# What is the rate of acceleration based on downward pitch?

In other words, I want to understand how gravity converts into thrust in respect to pitch. What is the general mathematical relationship? (For visuals, imagine a bird diving to gain speed.)

$a = g \cdot \sin(\theta)$
where $a$ is acceleration, $g$ is the free fall acceleration (9.8 m/s^2) and $\theta$ is the pitch angle down (0 assuming horizontal).