What's the tangential acceleration of a freely swinging pendulum when it makes an angle θ with the vertical?
a = F/M
The acceleration equals the component of the weight along the direction of motion (F), divided by M.
You may need to draw a figure to see the relationship between Weight (M g), Tension and F. The angle theta is used in calculating the components.
The accelerating force F is perpendicular to T, the pendulum string tension.