Think of a wave formed by a crown in a stadium,
    in which each spectator successively moves his or her arms up and down in response to a neighbor's motion,
    propagating a wave around the stadium perpendicular to the direction of the arm motion,
The wave travels from one side of the stadium to the other side
    much faster than any spectator could.

Usage：
    Green line: displacement from the equilibrium point.
    Yellow line: velocity of the medium
    Red line : acceleration

     Move the mouse to change the observed points.

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Author：Fu-Kwun Hwang, Dept. of physics, National Taiwan Normal University