I am having an issue understanding this mathematical modeling of an electromagnetic braking system.

I have this equation,

d^2z/dt^2 = g - (RLl^2/am*pi) * z^2/(z^2+4a^2)^5 * dz/dt, where z is the position and t the time

d^2z/dt^2 as far as I understand is the acceleration.

What I don't understand is why the acceleration (second derivative) is defined in terms of the velocity (first derivative)?

I am trying to graph the acceleration over time but I can't because I don't know the equation for velocity.

I hope I made myself clear; I'm having a hard time with this assignment.

I am having an issue understanding this mathematical modeling of an electromagnetic braking system.

I have this equation where z is the position and t the time,

$\frac{d^2z}{dt^2} = g - \frac{RL\lambda^2}{am\pi} \frac{z^2}{(z^2+4a^2)^5} \frac{dz}{dt}$

$\frac{d^2z}{dt^2}$ as far as I understand is the acceleration.

What I don't understand is why the acceleration (second derivative) is defined in terms of the velocity (first derivative)?

I am trying to graph the acceleration over time but I can't because I don't know the equation for velocity.

I hope I made myself clear; I'm having a hard time with this assignment.