An ideal simple pendulum consists of a particle suspended from a fixed point by means of a massless string. When the pendulum is at rest the string is vertical (as in position B in the diagram). If the particle is drawn aside carefully so that the string makes a small angle q with the vertical and then released, the pendulum oscillates with approximately simple harmonic motion in a vertical plane.

The pendulum used in this experiment consists of a metal spherical bob suspended from a fixed point via a light string. As the bob is not a point-like particle and the string has a small mass, the pendulum here is not an ideal simple pendulum. The pendulum can be considered as an ideal simple pendulum, however, provided that (1) the mass of the bob is much greater than the mass of the string and (2) the length of the string is much greater than the diameter of the bob.

The period of oscillation of a simple pendulum, that is the time required for the pendulum bob to perform one complete oscillation, for example, B→C→B→A→B, is given by the relation

                                                         T=2π√(L/g)                                           

where L is the length of the pendulum (that is, the distance from the fixed point to the centre of the bob) and g is the acceleration due to gravity. In this experiment the period of oscillation T is measured for different values of L.  The acceleration due to gravity may be determined by measurements on a graph of T ² versus L and also by direct calculation.

Note that one oscillation is the cycle of the pendulum’s motion starting from any initial position and coming back to the same position, moving in the same direction.

While it uses a different method to measure oscillations the equations used in this experiment are explained well in this guide.

To try the pendulum lab yourself try your hand at these online labs / simulations: 

Pendulum Lab

Simple Pendulum

Pendulum Lab

Pendulum Pocket Lab