In this image you can see a swinging pendulum and that there's a net force $A$ that causes it to move in circular motion. It is constantly updating velocity vector $v$ direction to be perpendicular with the direction towards the pivot point.
As I understand the net force $A$ should be $A = T - mg$, where $T$ is the tension force and is equal to $T = mg\cos\theta$. I tried to solve for net force $A$ but I'm not getting the correct result. In my calculations when $\theta = 0$, the net force is zero. But that's wrong because there's clearly always a force that affects the pendulum, since its velocity vector constantly changes its direction.
What am I missing?