so I’m quit new to simulations in python and I tried to simulate the cart and pendulum problem as shown here

I don’t know why but the animation is distorting the length of the pendulum rope.
It happened also with regular pendulum simulations so i finally decided to ask.

I checked the length of the rope and it doesn’t seem to change so i think the problem is with the animation proportions.

Relevant code block:

    def animate(i):
        ln.set_data([0, x1_vals[i], x2_vals[i]], [0, 0, y2_vals[i]])


    fig, ax = plt.subplots(1, 1, figsize = (8, 8))
    ax.get_xaxis().set_ticks([])
    ax.get_yaxis().set_ticks([])

    ln, = ax.plot([], [], 'r--', lw = 3, marker = 'o', markersize = 12)

    ax.set_xlim(min(min(x1_vals), min(x2_vals)),max(max(x1_vals), max(x2_vals)))
    ax.set_ylim(min(0, min(y2_vals)),max(0, max(y2_vals)))

    ani = animation.FuncAnimation(fig, animate, frames = 1001, interval = 50)
    ani.save('CartAndPendulum.gif', writer='pillow', fps = 25)

1. shorter rope

2. longer rope

3. Constant rope length demonstrated: