At what point of a pendulum's swing is its acceleration the greatest? What does this tell you about where the forces act in a pendulum?
Is my answer correct?
The force that causes the acceleration of the pendulum is due to the weight of the pendulum. The weight is constant but if the pendulum is still, hanging in a direct downwards position, then the weight does not make the pendulum swing. This is because the force is vertical and the pendulum's possible motion is horizontal when it's at the bottom. If we pull the pendulum some distance to the side and release it, then it will swing. That's because when the pendulum is taken to the side, a component of the weight acts in the direction that the pendulum can move - along the arc.
It's often convenient to take a vector (like the pendulum's weight) and resolve it into 2 components that are perpendicular. For a pendulum, the components of the weight that help us are along the string (or rod) and perpendicular to the string. A perpendicular to the string would be tangential to the arc of possible motion. Give the symbol Wt to the component that is tangential to the arc. When we take it to the side (so far that the string (or rod) was horizontal), then the weight would of course point downwards and that direction would be tangential to the arc of the pendulum's possible movement. In that case, Wt is equal to the weight. So, the entire weight provides force for the acceleration of the pendulum.
However big the swing is, the pendulum's acceleration is greatest when the component of the pendulum's weight lies tangential to the arc of possible motion.