關於simple pendulum

Before explaining the concept of simple pendulum, it should be highly mentioned that SHM in a simple pendulum can also be considered as a special case of circular motion. When we are to deal with questions involving circular motions, we must FIRST find the centre of gyration (i.e. the centre of the circular path). And then we consider the forces acting on the body as the NEXT STEP.

When we are to consider the forces acting on the pendulum, we should resolve forces RADIALLY and TRANSVERSELY (i.e. along and perpendicular to the radius direction respectively). In this case, the tension is along the radius direction, so we don't resolve the tension. Instead, since the weight is neither along nor perpendicular to the radius direction, so we reslove the weight radially and transversely. The weight along the transverse direction acts as the restoring force while the weight along the radial direction cancels the tension.

Hope the above information helps =)

The restoring force in driving the pendulum bob is actually the resultant force of the tension T and the weight of the bob mg.

If you try to draw out a force (vector) diagram for the two froces, T and mg, and find their resultant force from the geometry of the diagram, you would find that their resultant froce is given by mg.sin(theta), where theta is the angle between the vertical and the suspended string, and acts in the direction perpendicular to the string.