中心差分法

代码说明：

  运动方程中的速度向量和加速度向量用位移的某种组合来表示，将微分方程组的求解问题转化为代数方程组的求解问题，并在时间区间内求得每个微小时间区间的递推公式，进而求得整个时程的反应。 中心差分法是一种显示的积分法，它基于用有限差分代替位移对时间的求导（即速度和加速度）。(Velocity vectors and acceleration vectors in motion equations are expressed by a combination of displacements. The problem of solving differential equations is transformed into the problem of solving algebraic equations. The recurrence formulas of each small time interval are obtained in the time interval, and then the response of the whole time interval is obtained. The central difference method is a display integration method, which is based on the finite difference method instead of the displacement derivative of time (i.e. velocity and acceleration).)