The graph below is the solution to $y'' + 4 y = 0$ with initial conditions $y(0) = 1$ and $y'(0) = 0$. The solution is $y(t) = \cos(2t)$. This represents the undamped motion of a mass on a spring. You can modify, the code for different parameters. Press "Evaluate" to draw the animation. The animation is drawn with SAGE and can take a somewhat long time to draw.