1. Gravity (Weight): This force acts vertically downwards and is responsible for pulling the pendulum bob back towards its equilibrium position.

2. Tension in the String: This force acts along the string and acts to counteract the component of gravity that is perpendicular to the string.

Important Note: While these are the two main forces, there are other smaller forces that can also act on a pendulum, such as air resistance and friction at the pivot point. However, these forces are usually negligible in a simple pendulum model.