Abstract: A simple model of inverted pendulum is proposed and an approximate solution for the movement of the pendulum in the condition of small angle is given by means of combining theoretical analysis with numerical calculation of MATLAB. The result shows that there exists a critical frequency and that the inverted pendulum would fall down when the driving frequency is smaller than the critical frequency while the inverted pendulum may not fall down when the driving frequency is bigger than the critical frequency.

Key words: inverted pendulum, MATLAB, critical frequency