A simple pendulum (see Figure 1) is a simplified version of a 'real' pendulum where the following simplifications are assumed:

• 线程没有mass and cannot extend

• The mass attached to the thread is a point mass

• The motion is two-dimensional

• There is no friction or air resistance

Figure 1:A simple pendulum with thread length L, mass m, angular displacement θ

Here you can find the theory forderivation of oscillation period and gravitational acceleration.